1
Author:
Title:
Shoushounova, Timothiax
Air-brazing of Aluminum Oxide Ceramics Using Copper Powders
The accompanying research report is submitted to the University of Wisconsin-Stout, Graduate School in partial
completion of the requirements for the
Graduate Degree/ Major:
Research Advisor:
Rajiv Asthana, Ph.D.
Submission Term/Year:
Number of Pages:
MS Manufacturing Engineering
Spring, 2013
54
Style Manual Used: American Psychological Association, 6th edition
√ I understand that this research report must be officially approved by the Graduate School and that
an electronic copy of the approved version will be made available through the University Library
website
√ I attest that the research report is my original work (that any copyrightable materials have been
used with the permission of the original authors), and as such, it is automatically protected by the
laws, rules, and regulations of the U.S. Copyright Office.
√ My research advisor has approved the content and quality of this paper.
STUDENT:
NAME: Timothiax Shoushounova DATE: May 15, 2013
ADVISOR:
NAME Rajiv Asthana, Ph.D. DATE: May 15, 2013
This section to be completed by the Graduate School
This final research report has been approved by the Graduate School.
Director, Office of Graduate Studies:
DATE:
2
Shoushounova, Timothiax. Air-brazing of Aluminum Oxide Ceramics Using Copper
Powders
Abstract
The purpose of this research was to investigate the effects of sintering and brazing
parameters on the hardness, flexural strength, and metallurgical structure of bonded ceramic
joints. The secondary objective was to determine which sintering and brazing parametric
combinations warrant further examination, based on bonding characteristics observed.
Aluminum Oxide ceramic discs were joined together using copper powder through the
process of air-brazing. Numerous samples were bonded under different parametric combinations.
Sintering parameters included temperatures of either 1400°C or 1500°C, with times of either 2 or
4 hours. Brazing parameters included temperatures of either 850°C or 1085°C, with times of 15,
30, 45, or 60 minutes. Sample joints were then studied under a microscope, micro-hardness
tested, and flexural strength tested.
In this study, no single parametric combination produced a predominantly superior
overall joint. However, certain combinations exhibited encouraging characteristics. The joint
fabrication parameters that produced the most viable overall joints and those recommended for
further examination include the following six combinations: (1500°C for 4 hr/ 1085°C for 15
and 30 min), (1500°C for 4 hr/ 850°C for 30 and 45 min), and (1500°C for 2 hr/ 1085°C for 15
and 30 min.)
3
The Graduate School
University of Wisconsin-Stout
Menomonie, WI
Acknowledgments
I wish to thank Dr. Rajiv Asthana for his guidance and assistance throughout the course
of this project, for without his support and vast knowledge of the subject matter, this research
investigation would not have been possible.
Additionally, I wish to express my sincere appreciation to all of my professors at the
University of Wisconsin-Stout for all they have taught me throughout my studies.
4
Table of Contents
............................................................................................................................................. Page
Abstract ...................................................................................................................................... 2
List of Tables .............................................................................................................................. 6
List of Figures ............................................................................................................................. 7
Chapter I: Introduction ............................................................................................................... 8
Statement of the Problem ................................................................................................. 9
Purpose of the Study ........................................................................................................ 9
Assumptions of the Study ................................................................................................ 9
Definition of Terms ....................................................................................................... 10
Limitations of the Study ................................................................................................ 11
Chapter II: Literature Review .................................................................................................... 13
Techniques Used in Joining Ceramics ............................................................................ 13
Mechanical methods of joining ceramics................................................................... 14
Thermo-mechanical methods of joining ceramics ..................................................... 14
Liquid phase joining of ceramics .............................................................................. 15
Brazing Techniques Used to Join Ceramics ................................................................... 16
Chapter III: Methodology .......................................................................................................... 19
Material Composition .................................................................................................... 19
Sample Fabrication and Joint Construction .................................................................... 20
Sample Preparation for Examination and Testing ........................................................... 22
Data Collection Procedures............................................................................................ 23
Data Analysis ................................................................................................................ 24
5
Limitations .................................................................................................................... 24
Chapter IV: Results ................................................................................................................... 25
Sample Information ....................................................................................................... 25
Microscopic Examination of the Microstructure of Sample Joints .................................. 25
Structure and composition of representative sample joints ........................................ 25
Porosity measurements and observations .................................................................. 28
Brazing temperature observations ............................................................................. 30
Micro-hardness Measurements ...................................................................................... 31
Modulus of Rupture (Flexural Strength) Measurements ................................................. 34
Chapter V: Discussion............................................................................................................... 36
Limitations .................................................................................................................... 36
Conclusions ................................................................................................................... 37
Recommendations ......................................................................................................... 38
References ................................................................................................................................ 39
Appendix A: Chemical Properties of Aluminum Oxide and Copper ......................................... 41
Appendix B: Density Tables for Sintering Temperatures and Times ......................................... 42
Appendix C: Formulas Used in Research Calculations ............................................................. 46
Appendix D: Sample Sintering and Brazing Parameters ........................................................... 47
Appendix E: Micro-hardness Testing Data ............................................................................... 49
Appendix F: Modulus of Rupture (Flexural Strength) Data ...................................................... 53
6
List of Tables
Table 3.1: Polishing Procedure for Epoxy Mounted Sectioned Samples .................................... 22
Table 4.1: Average Porosity of Sintered Discs ........................................................................... 28
Table 5.1: Joint Fabrication Parameters Exhibiting the Best Bonding Characteristics ................ 38
Table A.1: Chemical Properties of Aluminum Oxide ................................................................. 41
Table A.2: Chemical Properties of Copper ................................................................................ 41
Table B.1: Density Calculations (Discs Sintered at 1400 °C for 2 hours) ................................... 42
Table B.2: Density Calculations (Discs Sintered at 1400 °C for 4 hours) ................................... 43
Table B.3: Density Calculations (Discs Sintered at 1500 °C for 2 hours) ................................... 44
Table B.4: Density Calculations (Discs Sintered at 1500 °C for 2 hours) ................................... 45
Table C.1: Formulas Used in Research Calculations .................................................................. 46
Table C.2: Modulus of Rupture Formula ................................................................................... 46
Table D.1: Sample Sintering Temperatures and Times & Brazing Temperatures and Times ...... 47
Table E.1: Average Length of Sample Micro-hardness Indentations .......................................... 49
Table E.2: Average Knoop Hardness of Sample Indentations .................................................... 51
Table F:1: Modulus of Rupture Calculations ............................................................................. 53
7
List of Figures
Figure 3.1: Parametric combination matrix for sintering and brazing conditions. ....................... 21
Figure 4.1: Magnification of sample #2 showing the microstructure of the joint.. ...................... 26
Figure 4.2: Magnification of sample #8 showing the microstructure of the joint.. ...................... 26
Figure 4.3: Magnification of sample #38 showing the microstructure of the joint.. .................... 27
Figure 4.4: Magnification of sample #50 showing the microstructure of the joint.. .................... 27
Figure 4.5: Interaction region comparison between samples #40 and #16.. ................................ 29
Figure 4.6: Epoxy penetration of sample #2 in the joint region.. ................................................ 30
Figure 4.7: Graph of average micro-hardness measured across joint.. ........................................ 32
Figure 4.8: Graph of average micro-hardness measured along the joint bottom boundary.. ........ 33
Figure 4.9: Graph of sample modulus of rupture (flexural strength).. ......................................... 34
8
Chapter I: Introduction
Ceramics are very difficult to machine. Ceramic parts are either made to net-shape or are
assembled from simpler units via joining. One method of joining ceramic parts is brazing.
Brazed ceramics are used in many applications, such as spark plugs, nozzles, cutting tools, and
connectors for the nuclear industry. The electronics industry utilizes brazing techniques to make
ceramic-metal and glass-metal seals for vacuum tubes and microwave reflectors. Implantable
electronics (e.g., neuro-stimulators) employ ceramic feedthroughs that are brazed using gold to
create hermetic seals.
“Due to different types of chemical bonding, brazing metals to ceramic surfaces presents
various challenges. The difference in the electron configurations prevents the wetting of liquid
metal on a ceramic surface and, therefore, prevents joining with high bond strength.” (Reichle,
Koppitz, & Reisgen, 2010) Successfully joining ceramics by brazing is normally done with
expensive processes that require specialized fluxes and elaborate high vacuum furnace systems.
“Titanium or zirconium is added to the brazing filler metal (BFM) for vacuum brazing processes.
These elements within the BFM react with the ceramic surface and form a metallic/ceramic
interlayer that improves the wetting of the BFM on the ceramic.” (Reichle et al., 2010)
An inexpensive alternative method used to bond select systems is a procedure known as
air-brazing. Air-brazing is a process of brazing in a non-vacuum, open atmosphere in which the
ceramic and filler metal are allowed to react with oxygen. One system for which air-brazing
works well is copper/alumina joints. For this reason, air-brazed joints consisting of high-purity
aluminum oxide (Al2O3)and commercial-purity copper powders were selected as the subject for
this research study. Bonding of alumina/copper/alumina joints were achieved by direct brazing in
air in both the copper solid state (850°C) and in the copper liquid state (1085°C).
9
Statement of the Problem
A need existed to study the relationships between alumina/copper joint fabrication
parameters and the bonding characteristics that result from air-brazing in order to explore
alternate ceramic brazing techniques and perhaps aid ceramic joint design for select system
applications.
Purpose of the Study
The purpose of this study was to investigate, examine, and compare the effect of ceramic
fabrication parameters (sintering temperature and time) and brazing parameters (brazing
temperature and time) on the metallurgical microstructure, micro-hardness, and flexural strength
of directly bonded alumina/copper/alumina joints. The secondary objective was to determine
which sintering and brazing parametric combinations warrant further, in depth examination,
based on the bonding characteristics observed in this study.
Assumptions of the Study
The University of Wisconsin-Stout, Manufacturing Engineering Department provided all
necessary materials, equipment, and funding required to proficiently accomplish the scope of this
project. The data collected and observations recorded through this investigation were not to be
interpreted as representative of a larger sample size or population. The samples produced in this
research project were saved for future examination by others including, but not limited to,
chemical analysis of compounds produced within the interaction regions of the bonded joints,
additional micro-hardness testing of the joints, and supplementary microscopic observations of
the reaction region and grain structure within the joints. The findings of this study were also
intended to assist in determining which joint fabrication parametric combinations warrant further
in-depth examination involving much larger sample sizes.
10
Definition of Terms
Alumina. “A white granular material that is refined from bauxite ore, a little finer than
table salt, properly called aluminum oxide.” (Alcoa, 2013)
Brazing. “A group of joining processes which produces a coalescence of materials by
heating them to a suitable temperature and by using a filler metal having a liquidus above 450°C
and below the solidus of the base metals.” (American Welding Society Committee on Brazing
and Soldering, p. ix)
Calcine. “To heat (a substance) to a high temperature but remain below the melting or
fusing point, causing loss of moisture, reduction or oxidation, and the decomposition of
carbonates and other compounds.” (The Free Dictionary, 2013)
Colloidal. “A system in which finely divided particles, which are approximately 10 to
10,000 angstroms in size, are dispersed within a continuous medium in a manner that prevents
them from being filtered easily or settled rapidly.” (The Free Dictionary, 2013)
Flexural Strength. Also known as the modulus of rupture, bend strength, or fracture
strength. A mechanical parameter for brittle material defined as a material's ability to resist
deformation under load. The flexural strength represents the highest stress experienced within a
material at its moment of rupture. It is measured in terms of stress ( ). (Mitchell, 2004, p. 416)
Flux. “A substance used to promote fusion (as of metals or minerals); especially : one (as
rosin) applied to surfaces to be joined by soldering, brazing, or welding to clean and free them
from oxide and promote their union.” (Merriam-Webster Online, 2013)
Indentation Test. “A type of hardness test in which a hardened indenter is forced against
a material under a fixed load. The size of the indentation indicates the hardness of the material. ”
(Tooling University, LLC, 2012)
11
Knoop Hardness Test. “A micro-hardness test that uses a small pyramid-shaped
diamond indenter and relatively light loads between 10 g and 1 kilogram. The Knoop indenter
has a long diagonal that is perpendicular to and 7 times the length of the short diagonal.”
(Tooling University, LLC, 2012)
Porosity. “The ratio of the volume of interstices of a material to the volume of its mass.”
(Merriam-Webster Online, 2013)
Sintering. “The thermal treatment of a powder or compact at a temperature below the
melting point of the main constituent, for the purpose of increasing its strength by a bonding
together of the particles.” (Powder Metallurgy - Sintering and Sintering Furnaces, 2002)
Wetting. “The behavior of a liquid when the liquid contacts a solid surface. Liquids with
poor wetting ability tend to form droplets, while liquids with good wetting ability tend to spread
out evenly over the solid surface area.” (Tooling University, LLC, 2012)
Limitations of the Study
This study was limited in the fact that only two samples were fabricated for each
parametric combination of sintering and brazing conditions. Given the time constraints of this
investigation and the lengthy fabrication process to produce each of the samples used in this
study, creating a large sample size was not feasible. Because of the small sample size used for
the study, the data collected and observations recorded through this investigation were not to be
construed as representative of a larger sample size or population.
Another limitation was the number of parametric combinations of joint fabrication
conditions that could be considered. Only two sintering temperatures, two sintering times, two
brazing temperatures, and four brazing times were studied. Again, this was a time constraint
issue.
12
An additional limitation of this examination was the inability of the researcher to conduct
chemical analyses of the coalescent compounds produced within the interaction regions of the
joints.
13
Chapter II: Literature Review
The literature review for this project focused primarily on the area of the ceramic joining
techniques with an emphasis placed on brazing techniques. A synopsis of general techniques
used for joining ceramics is presented, followed by a more in-depth review or brazing ceramics.
Techniques Used in Joining Ceramics
Ceramic materials are based largely on chemical mixtures of Boron, Carbon, Nitrogen,
Oxygen, Aluminum, Silicon, Titanium, and Zirconium. Well-known ceramics are alumina
(Al2O3), silicon nitride (Si3N4), silicon carbide (SiC) and zirconia (ZrO2).
This portion of the literature review emphasizes techniques utilized in joining ceramics.
Joining ceramic materials presents many challenges, whether attempting to join them to together
or to a metal or alloy. “The strong ionic or covalent bonding of the principal elemental atoms or
ions not only provides ceramics with their desirable properties, including strength and chemical
inertness, but also makes them relatively difficult to join.” (Bansal, 2005)
Joining ceramics to metals is particularly difficult. Two key problems present themselves
when joining ceramics to metals. “The first is the difference in chemical bonding between
engineering ceramics and metals,… The second issue is that of the mismatch in coefficient of
thermal expansion (CTE) between the engineering ceramic and the metal to which it is joined.
This is probably the more important, particularly when designing components and joints.”
(Fernie, 1999)
Since the inception of ceramics, many techniques for joining them have been developed.
“Techniques used to join ceramics, both to themselves and to metals include: bolting, screw
threads, shrink fitting, ultrasonic welding, anodic bonding, diffusion bonding, glass-ceramics,
brazing, and adhesives.” (Nicholas, 1998)
14
Mechanical methods of joining ceramics. It is commonly regarded that mechanical
methods of joining ceramics, such as bolting or screw threads, are not techniques to be
recommended for challenging or extreme conditions. Mechanical joining methods can be
expensive. “Drilling holes and threads into engineering ceramics is very costly.” (Nicholas,
1998) “Even when the majority of the work is carried out in the green state, there is still a need
for diamond machining to remove the majority of the surface defects and imperfections that
could act as critical flaws in service.” (Bansal, 2005)
It is also very unusual for ceramics to have screw threads incorporated into their design.
“When designing with ceramics, sharp changes in section and re-entry features such as those
which occur with screw threads should be avoided, because they tend to act as stress
concentrators.” (Nicholas, 1998) Bolting also leads to problems, even if the holes are
successfully drilled through the ceramic or introduced into the green ceramic before sintering.
Spark plugs are made using the principle of shrink fitting. This process involves fitting a
metal casing over a ceramic component at high temperatures. The metal casing contracts more
than the ceramic on cooling. “This contraction generates radial compressive forces in the ceramic
sufficient to maintain the integrity of the joint.” (Fernie, 1999)
“Although it is possible to join ceramics with mechanical means, it is to be strongly
discouraged for ceramics where there will be significant (> 100°C) temperature changes during
service.” (Messler, 2004)
Thermo-mechanical methods of joining ceramics. The technology of ultrasonic joining
has been investigated by several groups over the past two decades, most notably in Japan. “The
application of ultrasonic acoustic waves with frequencies typically in the range 15–20 kHz and
wavelengths in the range 23–30 mm combined with moderate pressure of 10–30 MPa and
15
joining times between 0.1 and 10 s constitutes ultrasonic joining.” (Mizutani, 2006) This method
of joining is usually carried out in air. “A relatively soft metal such as Al, Cu or Mg is required
in the form of a thin sheet (typically, 2 mm thick) to produce an adequate joint. Al2O3, ZrO2,
Si3N4 and SiC can all be joined to metals by this method.” (Messler, 2004)
Anodic bonding, also known as electrostatic bonding or field-assisted bonding, is an
important technology used in silicon devices. “It is particularly useful for producing hermetic
seals for devices such as pressure sensors.” (Messler, 2004) During anodic bonding, a voltage is
applied across the interface of the two materials to be bonded, such as silicon and Pyrex glass, at
a temperature where the glass becomes an electrical conductor. “In a commercial anodic bonding
system, a typical range of bonding conditions for joining 101.6 mm diameter Pyrex glass wafers
to silicon might be a bond temperature of 350–450°C, a voltage of 500-1000 V, a bonding time
of 6 min and an overall cycle time of 20 min.” (Nicholas, 1998)
“Diffusion bonding is the formation of a joint between two surfaces by the application of
heat and pressure, over a period of time to produce a solid-state joint.” (Nicholas, 1998) The
process is usually carried out either in a vacuum or in a controlled atmosphere furnace. For most
ceramics, diffusion bonding may involve two or more ceramic pieces, often with the objective of
making a more complex component. However, because diffusion rates are slow in ceramics,
even at high temperature, techniques are required to promote the diffusion.
Liquid phase joining of ceramics. By far, the most common techniques for joining
ceramics are done through the presence of a liquid phase. There are several different types of
liquid phase processes available. Two of the most common are glass-ceramics and brazing.
Liquid phase processing techniques are normally described in terms of the type of joining
medium. In nearly all cases, the joining material is placed between the surfaces to be joined, and
16
heated in order to be melted or cured to produce the bond. “Liquid phase bonding processes are
distinct from conventional welding processes in that there is either no melting, or very limited
melting, of the parent material.” (Fernie, 1999) Liquid phase bonding has many advantages. The
most important of which is the ability to completely fill the gap between the bonded surfaces
without leaving any porosity, even if the surfaces to be joined are relatively rough. “In general,
liquid phase joining is a fast process, but the interface created is usually the weakest link in the
component. This must be recognized and taken into consideration when designing joints for
actual components.” (Nicholas, 1998)
Glass-to-metal sealing has existed for well over a century. “It was initially developed for
the production of incandescent light bulbs and thermionic tubes for early electronics. Although
now a mature technology, it is still widely used in the same fields and has further expanded into
the microelectronics industry over the past 50 years. ” (Nicholas, 1998) Glass–ceramics combine
the superior strength of ceramics with the ease of glass processing. “Glass–ceramics are notably
stronger and more chemically durable than ordinary amorphous glasses and hence produce
higher strength seals.” (Fernie, 1999) In order to achieve good bonding and sealing between
either glasses or glass–ceramics and metals or alloys, it is essential to control the interface
chemistry and wetting between the dissimilar materials, the same as any other joining process.
Brazing Techniques Used in Joining Ceramics
The technique of brazing places a metal alloy in a paste, powder, foil or wire form
between the surfaces to be joined. The assembly is then heated to a temperature above 450°C, so
that the metal melts and reacts with the surfaces to form the bond. When brazing ceramics it is
usual to use a vacuum or controlled atmosphere furnace, although there are some exceptions to
this. In a good brazed joint there should be no voids. “Critical to all aspects of liquid phase
17
bonding is a fundamental knowledge of wetting and spreading. In the molten state, most pure
metals such as Sn, Au, Ni, Cu and Ag will not wet ceramics. Instead, they have a tendency to
‘ball up’ on the ceramic surface.” (Messler, 2004)
“The most commonly used braze family is that of the Ag-based alloys. These are also the
most widely researched family of braze materials. The standard alloy is the 72Ag–28Cu eutectic
composition alloy which melts at 780°C.” (Fernie, 1999) Although this alloy does not wet
ceramics, there are a variety of ways to overcome this problem. A reactive element such as Ti
can be incorporated into the braze to produce an active metal braze. “Alternatively, the ceramic
surface can be modified to render it wettable by the eutectic composition braze. This is referred
to as ‘metallization’.” (Messler, 2004) This technique is most commonly associated with alumina
and is used in electronic devices.
Ag-based alloys offer an excellent option for joining ceramics to themselves and to
metals. “By far the most widely studied system to date is the Ag–Cu–O system.” (Fernie, 1999)
Despite low temperature limitations, they remain the widest known and best understood family
of brazes.
A relatively new process referred to as reactive air brazing (RAB) is one of the most
recent innovations in ceramic joining. As the name suggests, the technique is carried out in air,
eliminating the need for the careful control of atmosphere. “RAB offers a potentially ideal
solution to the joining of ceramics in applications within the low-to-medium temperatures up to
750°C.” (Bansal, 2005) In the RAB technique, a noble metal is combined with a low melting
point metal oxide. “This process promotes wetting by lowering the liquid–vapor and solid–vapor
interfacial energies. Hence, the bonding surfaces become readily wettable by the filler metal.”
(Bansal, 2005)
18
In terms of microstructure, RAB joints appear to be relatively simple when joining
ceramics to themselves. “Brazing to alumina is possible when sufficient Cu2O is available
through the thermodynamically favorable reduction of CuO to Cu 2O at the temperature used for
air brazing.” (Bansal, 2005) The copper oxide migrates to the surface of the ceramic, leaving
behind a zone of pure Ag. “However, if the degree of chemical reaction is too high, a thick brittle
CuAlO2 reaction layer is produced at the braze/substrate interface which leads to bonds which
exhibit low four-point bend strengths of <100 MPa.” (Bansal, 2005)
19
Chapter III: Methodology
Commercial-purity copper powders and high-purity aluminum oxide (Al2O3) were used
to study the influence of ceramic fabrication parameters (sintering temperature and time) and
brazing parameters (brazing temperature and time) on the metallurgical microstructure, microhardness, and flexural strength of directly bonded alumina/copper/alumina joints.
This chapter includes detailed descriptions pertaining to material composition, sample
fabrication and joint construction, sample preparation for examination and testing, data
collection procedures, data analysis, and limitations.
Material Composition
High-purity, calcined, low soda, alumina powders (Al2O3) with a median diameter of
0.37 micrometers (µm) and molecular weight of 101.96 g/mol were aquired from Reynolds
Metals Co. The alumina was compacted under a 20,000 lb (89 kN) load for 20-25 seconds to
make ceramic discs (5-8 mm thickness, 25 mm dia) on a hydraulic press using a tool steel punch
and die and zinc stearate as a lubricant. Pressed discs were visually inspected and only those
which had a flat and smooth surface were approved for use by the researcher. If after pressing, a
disc was cracked, had an uneven or pitted surface, or had chipped or broken edges, it was
excluded from use and discarded. A total of 128 conforming discs were fabricated for the study.
Compacted discs were sintered in air in a programmable sintering furnace at 1400°C and
1500°C with sintering times of 2 hours and 4 hours followed by slow cooling (80°C/h) to room
temperature. For each temperature and time combination, 32 ceramic discs were sintered. The
density of sintered discs were characterized by weight and volume measurements from which
percent porosity was estimated. Density tables for sintering temperature and time combinations
may be referenced in appendix B: Tables B.1, B.2, B.3, and B.4.
20
The copper used for the braze was obtained from Aldrich Chemical Co. and consisted of
99% pure spheroidal copper powders with a median diameter of 10 µm and molecular weight of
63.55 g/mol. Chemical properties of aluminum oxide and copper applicable to this study may be
referenced in Appendix A: Tables A.1 and A.2.
Sample Fabrication and Joint Construction
Four discs sintered under the same conditions were used to fabricate two joined couples
for each brazing condition. The discs were cleaned with common household rubbing alcohol to
remove any dust or foreign substance prior to the application of copper powders between the
discs. Approximately 1.0 ± 0.05g of copper powder was measured out and evenly distributed
upon the surface of one disc. A second disc was placed directly on top of the copper powders
directly above, and radially aligned with, the lower disc. The combination of copper powders
sandwiched between two sintered discs comprised one couple, hereon referred to as a ‘sample’.
A box furnace was used to air-braze the samples at two temperatures (850°C and
1085°C) and four brazing times (15, 30, 45, and 60 min.). There were 32 different parametric
combinations of sintering and brazing conditions. The brazing time started once the box furnace
reached the conditional brazing temperature. After the allotted time was complete, the furnace
was cooled in stages to ensure uniform cooling for all samples. Stage one was set at 83% of the
brazing temperature, stage two at 65%, stage three at 37%, and stage four, the final stage, was
room temperature. Once stage one’s temperature was reached, stage two’s temperature was set,
and so on. At stage four, the oven was turned off and allowed to cool overnight.
Figure 3.1 illustrates the parametric combination matrix for all sintering and brazing
conditions. Two samples were fabricated for each parametric combination, resulting in a total of
64 samples.
21
Combination #
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
Sintering
Temperature
(°C)
1400
1400
1400
1400
1400
1400
1400
1400
1400
1400
1400
1400
1400
1400
1400
1400
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
Sintering Time
(hr)
2
2
2
2
2
2
2
2
4
4
4
4
4
4
4
4
2
2
2
2
2
2
2
2
4
4
4
4
4
4
4
4
Brazing
Temperature
(°C)
1085
1085
1085
1085
850
850
850
850
1085
1085
1085
1085
850
850
850
850
1085
1085
1085
1085
850
850
850
850
1085
1085
1085
1085
850
850
850
850
Brazing
Time
(min)
15
30
45
60
15
30
45
60
15
30
45
60
15
30
45
60
15
30
45
60
15
30
45
60
15
30
45
60
15
30
45
60
Figure 3.1. Parametric combination matrix for sintering and brazing conditions. Two samples
were fabricated for each condition.
Not all samples bonded during the brazing process. Some sintering and brazing
parametric combinations produced samples that did not bond upon removal from the brazing
22
furnace. Samples subject to these circumstances were addressed later in Chapter 4: Results:
Sample Information, and were excluded from futher processing for this study.
Sample Preparation for Examination and Testing
One sample from each parametric combination was sectioned on a low-speed diamond
saw. The cutting speed for all samples was 8 m/min and all samples were loaded and fixtured
uniformly on the saw throughout the cutting process. In order to polish the samples, one half of
the sectioned sample was mounted in a two-part epoxy and allowed to harden in a cylindrical
mold (30 mm dia, 30 mm thickness). Permanent sample numbers were etched into the top
surface of the epoxy upon removal of the samples from the molds. The sectioned flat surface of
the epoxy mounted samples were ground and polished on an auto-polisher using magnetic disks,
glycol based polycrystalline diamond suspension abrasives, and colloidal silica in a five step
polishing procedure which has been outlined in Table 3.1. Once polished to a mirror-like finish,
the sectioned surfaces of the samples were ready for examination under an optical microscope.
Table 3.1
Polishing Procedure for Epoxy Mounted Sectioned Samples
Step #
1
Disk Type
Cameo Platinum II
Lube
water
Abrasive
-
Load
(lb)
25
Speed
(rpm)
150
Duration
(min)
3
2
Cameo Silver
Blue
6 µm Diamond
25
100
6
3
Cameo White
Blue
3 µm Diamond
25
100
6
4
Cameo Grey
Blue
1 µm Diamond
25
100
10
5
Cameo Grey
-
Colloidal Silica
25
100
10
23
The remaining sample from each parametric combination pair which was not sectioned
and mounted in epoxy was left whole after brazing, labeled with its appropriate sample number,
and set aside for mechanical testing. No other preperation was required for testing these samples.
Data Collection Procedures
Microscopic observations of the polished samples using magnifications ranging between
x50 and x500 were documented and many images depicting joint characteristics of the samples
were recorded and stored as computer files. The microscopic examination of the microstructure
involved recording many observations regarding the porosity, structure, and composition of the
ceramic, copper, and interaction regions as well as noting evidence of coalescence in the joint.
The polished samples were then subjected to micro-hardness testing across the joint
region under a 500 g load using a Knoop micro-indenter. Micro-indentation measurements were
taken uniformly across the joint in three locations, equally spaced, along the length of the sample
joint. These micro-indentations were made in the ceramic zone on both sides of the joint, at the
joint boundary on both sides of the joint , and in the center of the copper region of the joint. The
indentation lengths were then converted to Knoop hardness values.
The sample from each parametric combination pair that was set aside for mechanical
testing was tested using a three-point bend test fixture mounted on a tension tester in order to
measure the modulus of rupture or flexural strength of the bonded joints. Load versus deflection
curves were recorded during compressive loading in the three point bending configuration from
which the fracture strength of the samples was determined.
24
Data Analysis
The micro-structure investigation resulted in a subjective comparative analysis between the
samples of this study. These data gave rise to a qualitative analysis since no numerical analysis
was possible for these observations.
The density of sintered discs were characterized by weight and volume measurements
from which percent porosity was estimated. Micro-indentation marks were measured so as to
calculate Knoop hardness values across sample joints. Load versus deflection curves were
recorded during compressive loading in the three point bend test in order to calculate the flexural
strength for sample joints. These data made it possible to perform quantitative numerical
analysis.
Limitations
A methodological limitation of this study was that some of the samples did not bond
during the brazing process. Since only two samples were fabricated for each sintering and
brazing parametric combination, it is not known whether the reason for unbonded samples was
due to procedural weaknesses or if this was due to other causes.
25
Chapter IV: Results
The purpose of this study was to investigate, examine, and compare combinations of
sintering and brazing parameters on the metallurgical microstructure, micro-hardness, and
flexural strength of directly bonded alumina/copper/alumina joints with the goal of determining
which fabrication parameters warrant further, in depth examination, based on the bonding
characteristics observed in this study.
Sample Information
Appendix D: Table D.1. lists all samples with their respective sintering temperatures and
times and brazing temperatures and times. Samples that were excluded from this study because
they did not bond during the brazing process were striked out on the table.
Twenty four of 64 samples were excluded for this reason: #5, #9, #11, #14, #19, #20,
#21, #22, #23, #24, #27, #29, #31, #32, #33, #34, #35, #36, #47, #48, #51, #52, #56, and #60. Of
the 24 samples that failed to bond, 20 of them had a brazing temperature of 850°C. This
indicated that this brazing parameter produced weak joints. This result was not unexpected since
this temperature is well below the melting point of copper at 1084°C (reference Appendix A).
Microscopic Examination of the Microstructure of Sample Joints
The microscopic examination of the microstructure involved recording many
observations regarding the porosity, structure, and composition of the ceramic, copper, and
interaction regions as well as noting evidence of coalescence in the joint.
Structure and composition of representative sample joints. Figures 4.1, 4.2, 4.3, and
4.4 show the structure and composition of the ceramic, copper, and interaction regions of four
representative sample joints for comparison. Each figure shows four views of the same joint
location at different powers of magnification as denoted in the lower right corner of each frame.
26
Figure 4.1. Magnification of sample #2 showing the microstructure of the joint. This sample was
sintered at 1400°C for 2 hours and brazed at 1085°C for 30 minutes. There is some visual
evidence of coalescence in this joint. The copper region shown consists of copper oxide only.
Figure 4.2. Magnification of sample #8 showing the microstructure of the joint. This sample was
sintered at 1400°C for 2 hours and brazed at 850°C for 30 minutes. There appears to be no visual
evidence of coalescence in this joint. The copper region shown consists of copper oxide only.
27
Figure 4.3. Magnification of sample #38 showing the microstructure of the joint. This sample
was sintered at 1500°C for 4 hours and brazed at 1085°C for 30 minutes. There is much visual
evidence of coalescence in this joint. The copper region shown consists of copper oxide only.
Figure 4.4. Magnification of sample #50 showing the microstructure of the joint. This sample
was sintered at 1500°C for 4 hours and brazed at 1085°C for 15 minutes. There is much visual
28
evidence of coalescence in this joint. The copper region shown consists of mostly copper metal
and some copper oxide.
A major flaw present in nearly all of the samples that did bond was that the joint’s top
boundary had almost no interaction region. In most of the samples, there was a void between the
ceramic and copper regions as seen in the upper left frame of figures 4.1, 4.3, and 4.4. Many
times, this void was filled with epoxy from when the samples were mounted for polishing. Those
samples that did not have a void present at the top boundary, had a sharp transition between the
ceramic and copper regions of the joint, such as that seen in the upper left frame of figure 4.2.
In all of the samples that bonded with an interaction region present, the joint’s bottom
boundary showed a much greater interaction region than the top boundary. This is especially
noticable in figures 4.3 and 4.4. The greater interaction at the bottom boundary of the joint was
attributed to fact that gravity played a role in the coalescence of the materials during the brazing
process. This indicates that the bottom region of the joint is stronger than the top region.
Porosity measurements and observations. The average porosity of the sintered discs
fabricated in this study are shown in Table 4.1. The formula used to calculate the porosity may
be found in Appendix C: Table C.1.
Table 4.1
Average Porosity of Sintered Discs
Sintering Temperature (°C)
1400
Sintering Time (hr)
2
Average Porosity (%)
28.1
1400
4
24.1
1500
2
15.7
1500
4
14.2
29
The discs sintered at 1500°C were much less porous than those sintered at 1400°C.
Among the 1500°C sintered discs, those sintered for 4 hours were slightly less porous than those
sintered for 2 hours. When comparing observations of joint composition and structure, it was
found that samples sintered at 1500°C generally displayed much more interaction and
coalescence than those sintered at 1400°C.
Figure 4.5. Interaction region comparison between samples #40 (left) and #16 (right). Both
magnified x200. Sintering temperatures were 1500°C for sample #40 and 1400°C for sample
#16. All other parameters were the same (sintered for 4 hours, brazed at 1085°C for 45 minutes).
Figure 4.5 illustrates that sample #40 has a much thicker interaction region and greater
coalescence than sample#16, even though all other fabrication parameters were the same. This
indicates that sintering temperatures of 1500°C produce stronger joints.
Also of note is the visual difference in porosity that is apparent in the sample’s ceramic
regions along the bottom of the images. The grey splotches on sample #16 are pools of epoxy
that penetrated the sample during the epoxy mounting process prior to polishing. Sample #40
does not exhibit any signs of epoxy penetration. Throughout the study, all samples sintered at
1400°C absorbed epoxy to some degree. The epoxy penetration of sample #2 as shown in figure
4.6 was typical.
30
Figure 4.6. Epoxy penetration of sample #2 in the joint region. The thin light grey line along the
joint bottom boundary is an interaction region.
Brazing temperature observations. Earlier in this chapter, it was indicated that because
the samples did not bond together, those brazed at 850°C produced weak joints. This was the
case for the most part, regardless of whether the samples were sintered at 1400°C or 1500°C. In
fact, only 12 of 32 samples brazed at this temperature bonded. This idea of fragile bonds was
subsequently supported through observations that most of the few samples that did bond at this
brazing temperature formed joints similar in appearance to that of figure 4.2. The exception was
those samples that were sintered at 1500°C for 4 hours. Not surprising, since these sintering
conditions have been indicated as those which form durable bonds. In contrast, samples brazed at
1085°C and sintered at 1500°C produced joints which visually appeared more robust, such as
those shown in figures 4.3 and 4.4.
Another observation concerning brazing temperatures was that many samples brazed at
1085°C experienced regions of copper melt within the joint. This brazing temperature was just
31
above the melting point of copper at 1084°C (reference Appendix A). It has not been confirmed
by visual examination if this had any effect on joint bonding strength.
Micro-hardness Measurements
Samples were micro-hardness tested across the joint region under a 500 g load using a
Knoop micro-indenter. Micro-indentation measurements were taken uniformly across the joint at
three locations evenly spaced along the entire length of the sample joint. Seven indentations were
made across the joint at each of the three location. Two on each sides of the joint in the ceramic
region, one on each side of the joint at the ceramic/ copper boundary, and one in the center of the
copper region. The indentation length measurements in each region were then compiled and
averaged for each joint location.
Table E.1 of Appendix E shows the average observed length of the indentations
(measured in µm) as well as the specific location of each indentation relative to the central
geometry of the joint. These indentation lengths were then converted to average Knoop microhardness values using the formula for Knoops found in Appendix C: Table C.1. The average
Knoop hardness values (measured in HK) derived from the indentation lengths are listed in
Appendix E: Table E.2.
The graph in Figure 4.7 was constructed using this data as a comparative cross sectional
representation of the average micro-hardness measured across the joint for each bonded sample
produced. In all samples the central copper region is quite soft compared to the ceramic regions
shown to either side of the central copper region. Examination of the graph revealed two general
levels of micro-hardness just on the ceramic side of the bottom joint boundary (indentation 6 on
Table E.2.); one in the 300-400 HK range and another in the 900-1000 HK range. Particular
consideration was given to the parametric combinations of samples that were grouped in the 900-
32
1000 HK range. The samples that made up this group were #28, #30, #38, #40, #44, #46, and
#50. Appendix D: Table D.1 was reference to determine fabrication parameters of these samples.
All of these samples were sintered at 1500°C, five were sintered for 4 hours, and five were
brazed at 1085°C. Curiously, all four brazing times were present in this group.
Average Knoop Hardness Measured Across the Joint
1200.0
+25µm +15µm +10µm
Indentation 1 2
3
0
4
-10µm -15µm -25µm
5
6 Indentation 7
Sample #2
Sample #4
1000.0
Sample #6
Sample #8
Sample #10
800.0
Sample #12
Knoop Hardness (HK)
Sample #16
Sample #18
600.0
Sample #26
Sample #28
Sample #30
Sample #38
400.0
Sample #40
Sample #42
Sample #44
200.0
Sample #46
Sample #50
Sample #54
0.0
Top of Joint
Center of Joint
Bottom of Joint
Sample #58
Sample #62
Sample #64
-200.0
Cross Sectional Representation of Sample Joint
Figure 4.7. Graph of average micro-hardness measured across the joint. The central copper
region of the joint is about 20-25 µm wide. The ±15 µm markings are approximately 5 µm into
the ceramic. Indentation mark locations are referenced from Appendix E: Table E.1.
33
Average Knoop Hardness Measured Along Joint Bottom Boundary
800.0
50
700.0
26
Knoop Hardness (HK)
600.0
40 42
28
44
46
54
30
500.0
38
400.0
16
62
300.0
2 4
200.0
8
6
10
64
58
18
12
100.0
0.0
14
20 22 24
32 34 36
48
52
56
60
Sample # (samples with no hardness value did not bond and were excluded
from the study)
Figure 4.8. Graph of average micro-hardness measured along the joint bottom boundary. This
graph was constructed using the average micro-hardness data measured at the bottom boundary
of the joint and the measurement recorded 5 µm into the ceramic of the joint bottom.
Figure 4.8 compared the average micro-hardness of the joint along the bottom boundary.
On this graph, seven samples had micro-hardness above 550 HK. The samples that comprised
this group were #26, #28, #40, #42, #44, #46, and #50. From Appendix D: Table D.1 all of these
samples were sintered at 1500°C, five were sintered for 4 hours, five were brazed at 1085°C, and
once again, all four brazing times were present for this group.
34
Five samples measured well on both micro-hardness graphs. They were #28, #40, #44,
#46, and #50. All were sintered at 1500°C, four were sintered for 4 hours, three were brazed at
1085°C, and three were brazed for 45 minutes.
Modulus of Rupture (Flexural Strength) Measurements
One sample from each parametric combination that bonded was fracture tested using a
three-point bend test fixture mounted on a tension tester in order to measure the modulus of
rupture (flexural strength) of the bonded joints. Load versus deflection curves were recorded
during compressive loading in the three point bending configuration from which the fracture
strength of the samples was determined.
Sample Modulus of Rupture (Flexural Strength)
140
49
Modulus of Rupture (MPa)
120
100
45
25
80
37
43
55
60
13
3
40
15 17
1
5
57 59
41
7
20
0
53
39
63
61
9 11
19 21 23
27 29 31 33 35
47
51
Sample # (samples with no strength value did not bond and were
excluded from the study)
Figure 4.9. Graph of sample modulus of rupture (flexural strength). This graph was constructed
using maximum load values recorded during compressive loading in a three point bend test.
35
The modulus of rupture of bonded samples is shown in Figure 4.9. The formula used to
calculate the modulus of rupture may be found in Appendix C: Table C.2. Complete calculations
of the modulus of rupture for the samples may be referenced in Appendix F: Table F.1.
From the graph in figure 4.9, six samples were found to possess a modulus of rupture
above 60 MPa. The samples included in this group were #25, #37, #43, #45, #49, and #55. From
Appendix D: Table D.1 all of these samples were sintered at 1500°C, four were sintered for 4
hours, three were brazed at 1085°C, and three were brazed for 30 minutes.
36
Chapter V: Discussion
Ceramics are very difficult to machine. Ceramic parts are either made to net-shape or are
assembled from simpler units via joining. One method of joining ceramic parts is brazing.
Brazed ceramics are used in many applications. Brazing ceramics normally requires elaborate
vacuum systems. Air-brazing is an inexpensive alternative to this process for certain materials.
One system of material bonding applications for which air-brazing works well is copper/alumina
joints. Because there was a need to explore the relationships between alumina/copper joint
fabrication parameters and the resulting bonding characteristics, air-brazed joints consisting of
aluminum oxide and copper were selected as the subject for this research project.
The purpose of this study was to investigate, examine, and compare the effect of ceramic
fabrication parameters and brazing parameters on the microstructure, hardness, and flexural
strength of copper-alumina joints. A secondary objective was to determine which fabrication
parameters warrant further examination, based on the findings of this study.
Limitations
This study was limited in the fact that only two samples were fabricated for each
parametric combination of sintering and brazing conditions due to time constraints and lengthy
sample fabrication procedures. Creating a large sample size for this study was not feasible.
Because of this, the investigation should not to be construed as representative of a larger sample
size or any sample population. Another limitation was that the parametric combination matrix of
joint fabrication conditions that could be studied was limited. Due to time constraints, only two
sintering temperatures, two sintering times, two brazing temperatures, and four brazing times
were studied. An additional limitation was the inability of the researcher to conduct chemical
analyses of the coalescent compounds produced within the interaction regions of the joints. A
37
methodological limitation of this study was that several samples did not bond during the brazing
process.
Conclusions
Twenty four of 64 samples were excluded from this study because they did not bond
during the brazing process. Of the 24 samples that failed to bond, 20 of them had a brazing
temperature of 850°C. In fact, only 12 of 32 samples brazed at this temperature produced a
bonded joint. This indicated that this brazing parameter produced weak joints. This idea of
fragile bonds was subsequently supported through observations that most of the few samples that
did bond at this brazing temperature formed joints that exhibited very little coalescence or
interaction region. The exception was those samples that were sintered at 1500°C for 4 hours.
In contrast, samples brazed at 1085°C bonded very well. Only four of 32 samples brazed
at this temperature did not produce a bonded joint. Additionally samples brazed at 1085°C and
sintered at 1500°C produced joint characteristics which visually appeared robust.
A major flaw present in nearly all of the samples that did bond was that the joint’s top
boundary had almost no interaction region. In most of the samples, there was a void between the
ceramic and copper regions. And those samples that did not have a void at the top boundary, had
a sharp transition between the ceramic and copper regions of the joint.
In all of the samples that bonded with an interaction region present, the joint’s bottom
boundary showed a much greater interaction region than the top boundary. The greater
interaction at the bottom boundary of the joint was attributed to fact that gravity played a role in
the coalescence of the materials during the brazing process.
Discs sintered at 1500°C were much less porous than those sintered at 1400°C. Among
the 1500°C sintered discs, those sintered for 4 hours were slightly less porous than those sintered
38
for 2 hours. When comparing observations of joint composition and structure, it was found that
samples sintered at 1500°C generally displayed much more interaction and coalescence than
those sintered at 1400°C.
After a detailed investigation regarding the effects of sintering and brazing parameters on
the hardness, flexural strength, and metallurgical structure of directly bonded copper/alumina
joints, it has been determined that the parametric combinations shown in Table 5.1 exhibit the
most viable and successful bonding characteristics.
Table 5.1
Joint Fabrication Parameters Exhibiting the Best Bonding Characteristics
Sintering Temp. (°C)
Sintering Time (hr)
Brazing Temp. (°C)
Brazing Time (min)
1500°C
4
1085°C
15
1500°C
4
1085°C
30
1500°C
4
850°C
30
1500°C
4
850°C
45
1500°C
2
1085°C
15
1500°C
2
1085°C
30
Recommendations
In this study, no single parametric combination produced a predominantly superior
overall joint. However, certain combinations exhibited encouraging characteristics. The joint
fabrication parameters that produced the most viable overall joints and those recommended for
further examination are shown in Table 5.1.
39
References
Alcoa. (2013). What is alumina? Retrieved April 20, 2013, from http://www.alcoa.com/alumina/
en/info_page/alumina_defined.asp
American Welding Society Committee on Brazing and Soldering. (1976). Brazing manual (3rd
ed.). Miami, FL: American Welding Society.
Asthana, R., & Singh, M. (2007). Joining of partially sintered alumina, titanium, Hastealloy and
C-SiC composite using Ag-Cu brazes. Journal of the European Ceramic Society, 28,
617-631.
Bansal, N. P. (2005). Handbook of ceramic composites. Boston, MA: Kluwer Academic
Publishers.
Fernie, J. A., & Hanson, W.B. (1999). Best practices for producing ceramic–metal bonds.
Industrial Ceramics, 19, 172–175.
Free dictionary, The. (2013). Calcine. Retrieved April 20, 2013, from http://www.thefree
dictionary.com/calcine
Free dictionary, The. (2013). Colloidal. Retrieved April 28, 2013, from http://www.thefree
dictionary.com/colloidal
Lee, S.K., & Tuan, W. H. (2013). Formation of CuAlO2 at the Cu/Al2O3 interface and its
influence on interface strength and thermal conductivity. International Journal of Applied
Ceramic Technology. Taipei, Taiwan: Author.
Merriam-Webster online. (2013). Flux. Retrieved April 7, 2013, from http://www.merriamwebster.com/dictionary/flux
Merriam-Webster online. (2013). Porosity. Retrieved April 20, 2013, from http://www.merriamwebster.com/dictionary/porosity
40
Messler, R. W. (2004). Joining of materials and structures. Oxford, England: Elsevier
Butterworth-Heinemann.
Mitchell, B. S. (2004). An introduction to materials engineering and science for chemical and
materials engineers. Hoboken, NJ: John Wiley & Sons, Inc.
Mizutani, T., Matsuhiro, K., & Yamamoto, N. (2006). Advanced structural ceramics. Ceramic
Society of Japan, 114, 905–910.
Nicholas, M. G. (1998). Joining processes. Dordrecht, Netherlands: Kluwer Academic
Publishers.
Pavlova, M. A., & Metelkin, I. I. (1987). Pressure brazing ceramics to metals with a copper
metal alloy. Welding International, 1(2), 168-170. Retrieved April 16, 2013, from
http://www.tandfonline.com/doi/abs/10.1080/09507118709452106#.UZA7t6LvuSq
Powder metallurgy - Sintering and Sintering furnaces. (2002). Definition of sintering. Retrieved
April 4, 2013, from http://www.azom.com/article.aspx?ArticleID=1725
Reichle, M.S., Koppitz, T., & Reisgen, U. (2010). Brazing metal to ceramic in an
oxygen-containing atmosphere. Brazing and Soldering Today. Retrieved April 7, 2013,
from http://www.aws.org/bsmc/wj0310-57.pdf
Tooling University, LLC. (2012). What is the definition of “indentation test”? Retrieved April 4,
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Tooling University, LLC. (2012). What is the definition of “Knoop hardness test”? Retrieved
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from http://www.toolingu.com/definition-660230-28856-wetting.html
41
Appendix A: Chemical Properties of Aluminum Oxide and Copper.
Table A.1
Chemical Properties of Aluminum Oxide
_____________________________________________
Formula: Al2 O3
Molar mass: 101.96 g/mol
Density: 3.95 g/cm³
Melting point: 3,762°F (2,072°C)
Boiling point: 5,391°F (2,977°C)
_____________________________________________
Table A.2
Chemical Properties of Copper
_____________________________________________
Formula: Cu
Molar mass: 63.55 g/mol
Density: 8.96 g/cm³
Melting point: 1,984°F (1,084°C)
Boiling point: 4,643°F (2,562°C)
_____________________________________________
42
Appendix B: Density Tables for Sintering Temperatures and Times.
Table B.1
Density Calculations (Discs Sintered at 1400 °C for 2 hours)
Disc
#
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
Dia.
(in)
1.087
1.075
1.070
1.065
1.068
1.068
1.064
1.071
1.065
1.071
1.073
1.064
1.066
1.065
1.069
1.070
1.075
1.078
1.076
1.065
1.066
1.066
1.064
1.068
1.073
1.085
1.077
1.071
1.078
1.044
1.029
1.065
r (in)
0.544
0.538
0.535
0.533
0.534
0.534
0.532
0.536
0.533
0.536
0.537
0.532
0.533
0.533
0.535
0.535
0.538
0.539
0.538
0.533
0.533
0.533
0.532
0.534
0.537
0.543
0.539
0.536
0.539
0.522
0.515
0.533
r
(cm)
1.38
1.37
1.36
1.35
1.36
1.36
1.35
1.36
1.35
1.36
1.36
1.35
1.35
1.35
1.36
1.36
1.37
1.37
1.37
1.35
1.35
1.35
1.35
1.36
1.36
1.38
1.37
1.36
1.37
1.33
1.31
1.35
h (in)
0.259
0.265
0.259
0.264
0.262
0.260
0.259
0.262
0.254
0.259
0.261
0.257
0.258
0.262
0.259
0.261
0.258
0.261
0.262
0.261
0.258
0.265
0.263
0.259
0.264
0.265
0.264
0.261
0.268
0.325
0.340
0.258
h
(cm)
0.66
0.67
0.66
0.67
0.67
0.66
0.66
0.67
0.65
0.66
0.66
0.65
0.66
0.67
0.66
0.66
0.66
0.66
0.67
0.66
0.66
0.67
0.67
0.66
0.67
0.67
0.67
0.66
0.68
0.83
0.86
0.66
Vol. Weight
(cm3) (g)
3.94
10.72
3.94
10.85
3.82
10.89
3.85
10.88
3.85
10.91
3.82
10.86
3.77
10.87
3.87
10.93
3.71
10.84
3.82
10.87
3.87
10.91
3.74
10.85
3.77
10.86
3.82
10.82
3.81
10.76
3.85
10.97
3.84
10.93
3.90
10.89
3.90
10.76
3.81
10.99
3.77
10.87
3.88
10.91
3.83
10.81
3.80
10.83
3.91
10.91
4.02
10.95
3.94
10.82
3.85
10.84
4.01
11.14
4.56
13.15
4.63
14.18
3.77
10.83
Average
Density
(g/cm3)
Density
(g/cm3)
2.72
2.75
2.85
2.82
2.84
2.85
2.88
2.83
2.92
2.84
2.82
2.90
2.88
2.83
2.82
2.85
2.85
2.79
2.76
2.88
2.88
2.81
2.82
2.85
2.79
2.73
2.75
2.81
2.78
2.88
3.06
2.88
2.84
(Volume and density formulas may be found in Appendix C, Table C.1.)
43
Table B.2
Density Calculations (Discs Sintered at 1400 °C for 4 hours)
Disc
#
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
Dia.
(in)
1.081
1.057
1.074
1.064
1.031
1.051
1.027
1.032
1.025
1.030
1.040
1.031
1.035
1.032
1.028
1.020
1.025
1.014
1.019
1.029
1.029
1.027
1.028
1.006
0.972
1.029
1.020
1.018
1.027
1.022
r
(in)
0.54
0.53
0.54
0.53
0.52
0.53
0.51
0.52
0.51
0.52
0.52
0.52
0.52
0.52
0.51
0.51
0.51
0.51
0.51
0.51
0.51
0.51
0.51
0.50
0.49
0.51
0.51
0.51
0.51
0.51
r
(cm)
1.37
1.34
1.36
1.35
1.31
1.33
1.30
1.31
1.30
1.31
1.32
1.31
1.31
1.31
1.31
1.30
1.30
1.29
1.29
1.31
1.31
1.30
1.31
1.28
1.23
1.31
1.30
1.29
1.30
1.30
h (in)
0.256
0.256
0.261
0.266
0.278
0.274
0.260
0.266
0.260
0.265
0.266
0.264
0.270
0.342
0.360
0.410
0.404
0.396
0.315
0.287
0.303
0.334
0.350
0.325
0.298
0.197
0.357
0.248
0.263
0.357
h
(cm)
0.65
0.65
0.66
0.68
0.71
0.70
0.66
0.68
0.66
0.67
0.68
0.67
0.69
0.87
0.91
1.04
1.03
1.01
0.80
0.73
0.77
0.85
0.89
0.83
0.76
0.50
0.91
0.63
0.67
0.91
Vol
Weight
3
(cm )
(g)
3.85
10.84
3.68
10.88
3.87
10.97
3.88
10.99
3.80
10.98
3.90
10.97
3.53
10.93
3.65
10.92
3.52
10.95
3.62
10.88
3.70
10.95
3.61
10.92
3.72
10.94
4.69
14.13
4.90
15.16
5.49
16.93
5.46
16.59
5.24
16.09
4.21
13.20
3.91
11.80
4.13
12.36
4.53
13.98
4.76
13.70
4.23
13.47
3.62
9.81
2.68
8.06
4.78
14.92
3.31
10.47
3.57
10.88
4.80
14.89
Average
Density
(g/cm3)
Density
(g/cm3)
2.82
2.96
2.83
2.84
2.89
2.82
3.10
2.99
3.11
3.01
2.96
3.02
2.94
3.01
3.10
3.08
3.04
3.07
3.14
3.02
2.99
3.08
2.88
3.18
2.71
3.00
3.12
3.17
3.05
3.10
3.00
(Volume and density formulas may be found in Appendix C, Table C.1.)
44
Table B.3
Density Calculations (Discs Sintered at 1500 °C for 2 hours)
Disc
#
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
Dia.
(in)
1.004
1.018
1.008
1.011
1.005
1.031
1.029
1.008
1.013
1.012
1.018
1.010
1.009
1.009
1.010
1.020
1.009
1.014
1.033
0.992
1.016
0.989
1.031
1.021
1.023
1.007
r
(in)
0.50
0.51
0.50
0.51
0.50
0.52
0.51
0.50
0.51
0.51
0.51
0.51
0.50
0.50
0.51
0.51
0.50
0.51
0.52
0.50
0.51
0.49
0.52
0.51
0.51
0.50
r
(cm)
1.28
1.29
1.28
1.28
1.28
1.31
1.31
1.28
1.29
1.29
1.29
1.28
1.28
1.28
1.28
1.30
1.28
1.29
1.31
1.26
1.29
1.26
1.31
1.30
1.30
1.28
h (in)
0.241
0.243
0.242
0.250
0.243
0.253
0.250
0.259
0.246
0.242
0.241
0.242
0.238
0.250
0.254
0.246
0.240
0.247
0.246
0.255
0.250
0.255
0.251
0.247
0.263
0.252
h
(cm)
0.61
0.62
0.61
0.64
0.62
0.64
0.64
0.66
0.62
0.61
0.61
0.61
0.60
0.64
0.65
0.62
0.61
0.63
0.62
0.65
0.64
0.65
0.64
0.63
0.67
0.64
Vol
(cm3)
3.13
3.24
3.16
3.29
3.16
3.46
3.41
3.39
3.25
3.19
3.21
3.18
3.12
3.28
3.33
3.29
3.14
3.27
3.38
3.23
3.32
3.21
3.43
3.31
3.54
3.29
Average
Density
(g/cm3)
Weight
(g)
10.75
10.84
10.78
10.88
10.81
10.79
10.88
10.96
10.85
10.86
10.84
10.89
10.79
11.03
10.84
10.87
10.51
10.94
10.82
10.93
10.89
10.97
10.84
10.89
10.93
10.98
Density
(g/cm3)
3.44
3.34
3.41
3.31
3.42
3.12
3.19
3.24
3.34
3.40
3.37
3.43
3.46
3.37
3.25
3.30
3.34
3.35
3.20
3.38
3.28
3.42
3.16
3.29
3.09
3.34
(Volume and density formulas may be found in Appendix C, Table C.1.)
3.33
45
Table B.4
Density Calculations (Discs Sintered at 1500 °C for 4 hours)
Disc
#
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Dia.
(in)
1.007
1.010
1.017
1.006
1.031
1.007
1.002
0.998
0.998
0.999
1.001
1.002
1.005
0.999
1.028
1.014
1.004
1.000
0.986
0.995
0.988
r
(in)
0.50
0.51
0.51
0.50
0.52
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0.51
0.51
0.50
0.50
0.49
0.50
0.49
r
(cm)
1.28
1.28
1.29
1.28
1.31
1.28
1.27
1.27
1.27
1.27
1.27
1.27
1.28
1.27
1.31
1.29
1.28
1.27
1.25
1.26
1.25
h (in)
0.255
0.255
0.240
0.241
0.249
0.253
0.241
0.244
0.249
0.241
0.248
0.245
0.249
0.243
0.250
0.244
0.245
0.247
0.255
0.253
0.251
h
(cm)
0.65
0.65
0.61
0.61
0.63
0.64
0.61
0.62
0.63
0.61
0.63
0.62
0.63
0.62
0.64
0.62
0.62
0.63
0.65
0.64
0.64
Vol
(cm3)
3.33
3.35
3.19
3.14
3.41
3.30
3.11
3.13
3.19
3.10
3.20
3.17
3.24
3.12
3.40
3.23
3.18
3.18
3.19
3.22
3.15
Average
Density
(g/cm3)
Weight
(g)
10.84
11.07
10.74
10.88
10.89
10.84
10.80
10.79
11.10
10.92
10.87
11.02
10.92
10.87
10.79
10.86
10.94
10.83
10.94
10.93
10.92
Density
(g/cm3)
3.26
3.31
3.36
3.47
3.20
3.28
3.47
3.45
3.48
3.53
3.40
3.48
3.37
3.48
3.17
3.36
3.44
3.41
3.43
3.39
3.46
(Volume and density formulas may be found in Appendix C, Table C.1.)
3.39
46
Appendix C: Formulas Used in Research Calculations.
Table C.1
Formulas Used in Research Calculations
List of Formulas
Volume (V) = π (r2)(h)
Density (ρ) = m / V
Porosity (%) = [(ρ(theoretical) - ρ (calculated)) / ρ(theoretical)](100%)
Knoops (HK) = 14230 x (F/d2)
[F: gf, d: µm]
Table C.2
Modulus of Rupture Formula
Modulus of Rupture
Formula:
Column1
Column2
M = ( 3)(F)(L ) / ( 2)(b)(d2 )
M = Modulus Of Rupture (Pa or N/m2)
F = Breaking Load (lbf)
L = Distance Between Knife Edges on which the Sample is
Supported (in)
b = Average Specimen Width (in)
d = Average Specimen Depth (in)
Unit Conversion Factors:
1 m = 39.37 in
1 N = 0.2248 lbf
1 Pa = 1 N/m2
1 MPa = 106 Pa
47
Appendix D: Sample Sintering and Brazing Parameters.
Table D.1
Sample Sintering Temperatures and Times & Brazing Temperatures and Times
Sample
#
1
2
3
4
5
6
7
8
9
10
11
Sintering
Temperature (°C)
1400
1400
1400
1400
1400
1400
1400
1400
1400
1400
1400
Sintering
Time (hr)
2
2
2
2
2
2
2
2
2
2
2
Brazing
Temperature (°C)
1085
1085
1085
1085
1085
1085
850
850
850
850
850
Brazing Time
(min)
30
30
45
45
60
60
30
30
45
45
60
12
13
14
1400
1400
1400
2
4
4
850
1085
1085
60
30
30
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
1400
1400
1400
1400
1400
1400
1400
1400
1400
1400
1500
1500
1500
1500
1500
1500
1500
1500
4
4
4
4
4
4
4
4
4
4
2
2
2
2
2
2
2
2
1085
1085
1085
1085
850
850
850
850
850
850
1085
1085
1085
1085
1085
1085
850
850
45
45
60
60
30
30
45
45
60
60
30
30
45
45
60
60
30
30
48
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
2
2
2
2
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
850
850
850
850
1085
1085
1085
1085
1085
1085
850
850
850
850
850
850
1085
1085
850
45
45
60
60
30
30
45
45
60
60
30
30
45
45
60
60
15
15
15
52
53
54
55
56
57
58
59
60
61
62
63
64
1500
1500
1500
1500
1500
1400
1400
1400
1400
1400
1400
1400
1400
4
2
2
2
2
4
4
4
4
2
2
2
2
850
1085
1085
850
850
1085
1085
850
850
1085
1085
850
850
15
15
15
15
15
15
15
15
15
15
15
15
15
Samples that are struck through on this table were excluded from the study because they did not
bond during the brazing process.
49
Appendix E: Micro-hardness Testing Data.
Table E.1
Average Length of Sample Micro-hardness Indentations
Avg
Avg
Avg
Avg
Avg
Avg
Avg
Length
Length
Length
Length
Length
Length
Length
Sample
Indent 1
Indent 2
Indent 3
Indent 4
Indent 5
Indent 6
Indent 7
#
(µm)
(µm)
(µm)
(µm)
(µm)
(µm)
(µm)
2
137.0
137.8
241.7
254.4
237.7
145.6
147.6
4
150.3
157.8
334.0
251.9
231.9
143.9
143.3
6
165.2
162.6
259.7
281.1
231.8
188.1
191.8
8
143.2
144.0
311.2
281.0
318.7
146.9
149.9
10
146.9
144.3
408.1
308.5
267.6
143.1
147.7
12
130.3
135.0
384.7
335.9
282.3
154.9
154.6
14
no data
no data
no data
no data
no data
no data
no data
16
122.1
113.3
293.0
263.2
225.6
109.1
121.8
18
149.5
209.2
402.8
274.9
266.1
152.9
162.0
20
no data
no data
no data
no data
no data
no data
no data
22
no data
no data
no data
no data
no data
no data
no data
24
no data
no data
no data
no data
no data
no data
no data
26
97.4
99.5
258.7
312.7
153.8
87.6
87.9
28
95.0
96.9
273.3
317.6
182.4
88.6
85.8
30
104.5
94.1
287.6
262.6
258.1
89.6
86.6
32
no data
no data
no data
no data
no data
no data
no data
50
34
no data
no data
no data
no data
no data
no data
no data
36
no data
no data
no data
no data
no data
no data
no data
38
81.2
87.1
366.1
321.6
247.3
94.6
94.3
40
88.1
87.9
277.5
357.5
182.9
84.8
83.3
42
90.5
94.0
267.3
260.9
134.5
94.7
92.2
44
79.4
83.6
262.0
422.4
239.3
85.0
82.4
46
87.3
87.4
494.6
349.7
182.8
86.5
87.6
48
no data
no data
no data
no data
no data
no data
no data
50
87.0
84.8
271.3
355.1
135.6
85.7
88.1
52
no data
no data
no data
no data
no data
no data
no data
54
99.7
98.7
171.0
280.7
182.3
94.6
95.7
56
no data
no data
no data
no data
no data
no data
no data
58
153.8
155.3
365.9
287.0
263.9
149.0
141.9
60
no data
no data
no data
no data
no data
no data
no data
62
126.8
128.7
254.5
238.5
209.8
135.2
136.9
64
136.1
137.1
364.0
304.5
285.5
144.3
146.7
Indentation # 1 was measured 15 µm above top Ceramic/Copper Boundary (µm)
Indentation # 2 was measured 5 µm above top Ceramic/Copper Boundary (µm)
Indentation # 3 was measured at top Ceramic/Copper Boundary (µm)
Indentation # 4 was measured at Center of Copper Region (µm)
Indentation # 5 was measured at bottom Ceramic/Copper Boundary (µm)
Indentation # 6 was measured 5 µm below bottom Ceramic/Copper Boundary (µm)
Indentation # 7 was measured 15 µm below bottom Ceramic/Copper Boundary (µm)
51
Table E.2
Average Knoop Hardness of Sample Indentations
Avg
Avg
Avg
Avg
Avg
Avg
Avg
Knoop
Knoop
Knoop
Knoop
Knoop
Knoop
Knoop
Hardness
Hardness
Hardness
Hardness
Hardness
Hardness
Hardness
Sample
Indent 1
Indent 2
Indent 3
Indent 4
Indent 5
Indent 6
Indent 7
#
(HK)
(HK)
(HK)
(HK)
(HK)
(HK)
(HK)
2
379.3
374.5
121.8
109.9
126.0
335.5
326.6
4
314.8
285.9
63.8
112.1
132.3
343.8
346.5
6
260.7
269.2
105.5
90.1
132.5
201.1
193.3
8
347.1
343.3
73.5
90.1
70.1
329.7
316.6
10
329.7
341.5
42.7
74.7
99.4
347.6
326.3
12
419.1
390.2
48.1
63.1
89.3
296.4
297.7
14
no data
no data
no data
no data
no data
no data
no data
16
477.2
554.3
82.9
102.7
139.8
598.1
479.9
18
318.2
162.5
43.9
94.2
100.5
304.3
271.2
20
no data
no data
no data
no data
no data
no data
no data
22
no data
no data
no data
no data
no data
no data
no data
24
no data
no data
no data
no data
no data
no data
no data
26
749.5
718.7
106.3
72.7
300.9
927.9
920.2
28
787.8
758.3
95.3
70.6
213.9
907.1
965.7
30
651.5
802.9
86.0
103.2
106.8
885.6
948.0
32
no data
no data
no data
no data
no data
no data
no data
52
34
no data
no data
no data
no data
no data
no data
no data
36
no data
no data
no data
no data
no data
no data
no data
38
1080.0
937.9
53.1
68.8
116.3
794.5
800.7
40
916.7
920.9
92.4
55.7
212.8
990.2
1025.4
42
868.1
804.7
99.6
104.5
393.3
793.4
837.0
44
1128.6
1018.0
103.7
39.9
124.3
984.0
1047.1
46
933.6
931.4
29.1
58.2
213.0
950.9
926.5
48
no data
no data
no data
no data
no data
no data
no data
50
939.3
990.2
96.7
56.4
386.8
968.8
917.4
52
no data
no data
no data
no data
no data
no data
no data
54
716.3
729.9
243.3
90.3
214.0
795.6
776.9
56
no data
no data
no data
no data
no data
no data
no data
58
300.7
295.1
53.1
86.4
102.2
320.6
353.5
60
no data
no data
no data
no data
no data
no data
no data
62
442.3
429.4
109.9
125.1
161.7
389.2
379.6
64
384.0
378.5
53.7
76.8
87.3
341.7
330.5
Indentation # 1 was measured 15 µm above top Ceramic/Copper Boundary (µm)
Indentation # 2 was measured 5 µm above top Ceramic/Copper Boundary (µm)
Indentation # 3 was measured at top Ceramic/Copper Boundary (µm)
Indentation # 4 was measured at Center of Copper Region (µm)
Indentation # 5 was measured at bottom Ceramic/Copper Boundary (µm)
Indentation # 6 was measured 5 µm below bottom Ceramic/Copper Boundary (µm)
Indentation # 7 was measured 15 µm below bottom Ceramic/Copper Boundary (µm)
53
Appendix F: Modulus of Rupture (Flexural Strength) Data.
Table F.1
Modulus of Rupture Calculations
Sample
Sample
Max
Support
Sample
Fixture
Diameter,
Thickness,
Load,
Span,
MOR
MOR
#
Type
D (m)
H (m)
F (N)
L (m)
(N/m2)
(MPa)
1
Two Bar
0.0274
0.0137
1694.8
0.0142
2.4E+07
24
3
Two Bar
0.0272
0.0138
2686.8
0.0142
3.7E+07
37
5
N/A
N/A
N/A
N/A
N/A
N/A
N/A
7
Two Bar
0.0271
0.0139
1476.9
0.0142
2.0E+07
20
9
N/A
N/A
N/A
N/A
N/A
N/A
N/A
11
N/A
N/A
N/A
N/A
N/A
N/A
N/A
13
Disc Holder
0.0262
0.0140
2597.9
0.0192
4.6E+07
46
15
Two Bar
0.0264
0.0141
2544.5
0.0142
3.3E+07
33
17
Two Bar
0.0268
0.0144
2744.7
0.0142
3.3E+07
33
19
N/A
N/A
N/A
N/A
N/A
N/A
N/A
21
N/A
N/A
N/A
N/A
N/A
N/A
N/A
23
N/A
N/A
N/A
N/A
N/A
N/A
N/A
25
Disc Holder
0.0257
0.0132
3492.0
0.0192
7.5E+07
75
27
N/A
N/A
N/A
N/A
N/A
N/A
N/A
29
N/A
N/A
N/A
N/A
N/A
N/A
N/A
31
N/A
N/A
N/A
N/A
N/A
N/A
N/A
33
N/A
N/A
N/A
N/A
N/A
N/A
N/A
54
35
N/A
N/A
N/A
N/A
N/A
N/A
N/A
37
Disc Holder
0.0256
0.0130
3118.3
0.0192
6.9E+07
69
39
Disc Holder
0.0255
0.0130
1975.1
0.0192
4.4E+07
44
41
Disc Holder
0.0258
0.0133
1143.2
0.0192
2.4E+07
24
43
Disc Holder
0.0252
0.0140
3603.2
0.0192
6.3E+07
63
45
Disc Holder
0.0258
0.0133
3612.1
0.0192
7.5E+07
75
47
N/A
N/A
N/A
N/A
N/A
N/A
N/A
49
Disc Holder
0.0257
0.0128
5458.2
0.0192
1.3E+08
128
51
N/A
N/A
N/A
N/A
N/A
N/A
N/A
53
Disc Holder
0.0257
0.0129
2113.0
0.0192
4.8E+07
48
55
Disc Holder
0.0261
0.0133
2927.0
0.0192
6.1E+07
61
57
Two Bar
0.0263
0.0157
2709.1
0.0142
2.5E+07
25
59
Disc Holder
0.0261
0.0180
3323.0
0.0192
2.8E+07
28
61
Disc Holder
0.0261
0.0181
2010.7
0.0192
1.7E+07
17
63
Two Bar
0.0266
0.0159
2891.5
0.0142
2.6E+07
26
Length of Disc Holding Fixture (inches):
Length of Two Bar Fixture (inches):
Position Rate:
0.1 in/min to 0.5 lbf
0.05 in/min to sample break
set to stop at 50% max load drop off
0.754
0.560