R. A. L. Drew

Joining of engineering ceramics

Abstract Engineering ceramics such as alumina, zirconia, silicon nitride and silicon carbide can now be manufactured reliably with reproducible properties. As such, they are of increasing interest to industry, particularly for use in demanding environments, where their thermomechanical performance is of critical importance, with applications ranging from fuel cells to cutting tools. One aspect common to virtually all applications of engineering ceramics is that eventually they must be joined with another material, most usually a metal. The joining of engineering ceramics to metals is not always easy. There are two main considerations. The first consideration is the basic difference in atomic bonding: the ionic or covalent bonding of the ceramic, compared to the metallic bond. The second consideration is the mismatch in the coefficient of thermal expansion. In general, ceramics have a lower coefficient of thermal expansion than metals and, if high tensile forces are produced in the ceramic, either as a consequence of operating conditions or from the joining procedure itself, failure can occur. The plethora of joining processes available will be reviewed in this article, placing them in context from both an academic and commercial perspective. Comment will be made on research reporting advances on known technology, as well as introducing ‘newer’ technologies developed over the last 10 years. Finally, reviews and commentary will be made on the potential applications of the various joining processes in the commercial environment.

Wettability and spreading kinetics of Al and Mg on TiC

The wetting of TiC by liquid aluminum and magnesium under static argon between 800 and 1000 C is studied using the sessile drop technique. Extensive interfacial reaction occurs between Al and TiC, leading to the formation of aluminum carbide; conversely no reaction occurs for Mg/TiC. 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Preparation of nickel-coated powders as precursors to reinforce MMCs

The preparation of nickel-coated ceramic particles as precursors for MMC fabrication was studied. Al2O3 and SiC powders of three different particle sizes were successfully coated with Ni using an electroless metal plating technique. Uniform and continuous nickel films were deposited on both, alumina and silicon carbide powders, with a final composition ranging from 1.6 to 1.9wt% phosphorus, 18–21wt% of metallic nickel and the balance is ceramic. XRD showed that the Ni-P deposit was predominantly amorphous. However, after heat treatment, the metallic deposits crystallize into Ni and Ni3P phases, as confirmed by DSC analyses. Preliminary results showed that the use of Ni-coated powders enhances the wettability between the matrix and ceramic phase when processing particulate MMCs by infiltration techniques. The coating promoted easy metal flow through the preform, compared to the non-infiltration behavior of the uncoated counterpart samples.

Modeling the infiltration kinetics of molten aluminum into porous titanium carbide

Capillary-induced melt infiltration is an attractive method of fabricating metal/ceramic composites, as it offers the advantage of producing material with a high ceramic content and near-net-shape fabrication, without the use of an external force. In this work, the kinetics of infiltration of molten Al in TiC preforms, having a pore size of approximately 1 µm and porosity ranging from 20 to 40 pct, were investigated. The rate of infiltration was continuously monitored using a Thermo-Gravimetric analyzer (TGA), which measured the weight change of the preform as the metal intruded the sample. Infiltration profiles where generated over a temperature range of 860 °C to 1085 °C. At lower temperatures, an incubation period was evident in the profiles. The average activation energy for the different preforms was 90 kJ/mol, indicating that some form of mass-transfer mechanism was involved in driving the process. Furthermore, sessile drop tests showed an unstable wetting angle over a long period of time. Such wetting kinetics were responsible for the incubation period during the infiltration. The infiltration rate was also seen to be slower as the preform density increased. This was due to the tortuous nature of the channels and was characterized using curves obtained for liquids infiltrating the same preforms at room temperature. Both the tortuosity and the unstable contact angle have to be considered when modeling the infiltration kinetics of such a system. The existing model was therefore modified by incorporating terms to describe the process more accurately. A good correlation with the experimental data was seen to exist.

Improving the tribological behavior of copper through novel Ti–Cu intermetallic coatings

Abstract In order to improve the tribological properties of copper substrate, Ti–Cu intermetallic coatings were formed onto the surface of an oxygen free high conductivity (OFHC) copper by a pack cementation process. Evaluation of coated and uncoated samples carried out by a laboratory tribo-tester under dry conditions indicated significant improvements in tribological behavior of the coated samples. It was observed that the coefficient of friction was decreased from 0.8 in the uncoated copper to about 0.15 for the coated samples. Microstructural examination revealed that the specimen surface zone consists of three layers; the outermost layer is the transformation product of β phase containing α-Ti and Ti 2 Cu, the intermediate layer is composed of intermetallic compounds including the TiCu and Ti 2 Cu phases, and the innermost layer is a solid solution of titanium in copper. Accordingly, plastic deformation of the wear surfaces in coated samples decreased due to the presence of these intermetallic coatings. Study of wear surfaces, wear debris and counterface surfaces suggested delamination and oxidation wear to be the predominant wear mechanisms in the coated samples.

Microstructure and heat-treatment response of Al-2024/TiC composites

Abstract Composites of a commercial Al-2024 alloy reinforced with particulate TiC have been produced by the pressureless melt infiltration route. Infiltration was carried out at 1200°C for 2 h and composite materials containing 52 and 55 vol.% TiC were obtained. The phases other than the matrix and the reinforcement were identified as CuAl2, AlTi3, Ti3AlC, and TiAl3. It was found that the composites were age hardenable after solutionizing at 530°C for 150 min and subsequent natural or artificial ageing at 190°C. Mechanical properties of the composites, as-fabricated and heat-treated, showed a strong dependency on the ceramic content. After heat treatment, the 55% TiC composites showed an increase of hardness from 28.5 to 38.5 HRC; meanwhile ultimate tensile strength increased from 379 to 480 MPa. A similar behavior, as a function of heat treatment, was observed for the composites containing 52% TiC, but the estimated values were lower than those obtained for the 55% TiC composites. Scanning electron microscope (SEM) examination revealed transgranular fracture throughout the CuAl2 agglomerated precipitates in the as-fabricated composites. In the heat-treated conditions, brittle and intergranular fractures were observed.

Small punch testing for assessing the tensile strength of gradient Al/Ni-SiC composites

Abstract The versatility of the shear punch test (SPT) was considered as an alternative for assessing the tensile properties of Al/Ni–SiC gradient composites fabricated by infiltration of liquid aluminum into coated ceramics. Tensile strength values of the order of 220 to 250 MPa were obtained. The nature of the shear-fractured surfaces was irregular for the composite specimens, with shear failure over most of the surfaces but with some tensile failure at the margins. The substantial accumulation of NiAl 3 in the upper section specimen resulted in a higher strength and brittleness of the matrix compared to the low nickel content matrix sections.

Melt extraction processing of structural Y2O3–Al2O3 fibers

Compounds in the system Y2O3-Al2O3 are promising materials for optical, electronic and structural applications. In this study, a melt extraction process with a new approach to making ceramic fibers was used to produce amorphous fibers in the Y2O3‐Al2O3 system within the 20‐30-micron size range. Smooth and uniform cross section fibers with relatively high tensile strength were obtained depending on the wheel velocity. X-ray diAraction of as-extracted fibers revealed the non-crystalline nature of the yttriaalumina compositions. The crystallization and glass transition temperatures of non-crystalline fibers were determined using diAerential thermal analysis (DTA). Crystalline phases were identified by X-ray diAraction in the fibers after heat treatment. # 2000 Elsevier Science Ltd. All rights reserved.

Microstructural development during diffusion bonding of α-silicon carbide to molybdenum

Abstract Silicon carbide was joined to molybdenum by solid state bonding at temperatures ranging from 1200 °C to 1700 °C. The interfaces were characterized by scanning electron microscopy, electron probe microanalysis, and X-ray diffraction. Diffusion of Si and C into Mo resulted in a reaction layer with two main phases: Mo 5 Si 3 and Mo 2 C. For temperatures higher than 1400 °C a ternary phase of composition Mo 5 Si 3 C was also formed, and at 1700 °C nucleation of MoC was observed.

Wettability of TiC by commercial aluminum alloys

The effect of alloying elements on the wettability of TiC by commercial aluminum alloys (1010, 2024, 6061 and 7075) was investigated at 900°C using a sessile drop technique. Wetting increased in the order 6061 < 7075 < 2024 < 1010 for both, static argon or vacuum atmospheres. Alloys 1010 and 2024 wet TiC under both atmospheres, leading to contact angles in the order of 60° and less, while 7075 only wets under vacuum, with the poorest wettability being exhibited by 6061. Evaporation of Zn and Mg under vacuum conditions contributed to the rupture of the oxide film covering the aluminum drop and thereby improving wetting and spreading. Continuous and isolate Al4C3 was detected in all the cases. CuAl2 precipitation at the interface slightly decreased Al4C3 formation and increased the adhesion of 2024 to TiC.