synthesis methods of brazing TSP diamond to tungsten carbide and other dissimilar materials enable many more applications and longer life for tools.
Novel Methods of
BRAZING
Dissimilar Materials Novel microwave and combustion synthesis methods of brazing TSP diamond to tungsten carbide and other dissimilar materials enable many more applications and longer life for tools. C. Suryanarayana* John J. Moore* Colorado School of Mines, Golden, Colorado
Robert P. Radtke** Technology International Inc. Kingwood, Texas
T
ools made from ceramics have been central to the evolution, growth, prosperity, and quality of human life since the beginning of history. In fact, the first ceramic tools can be dated back as far as the Upper Paleolithic period. Sharp tools for hunting and cutting were readily made because of their brittleness. It is reported that the Chinese knew the art of drilling several hundred feet through strong limestone way back in 1700 BC. They were able to achieve this by pounding a single diamond stone into a suitable brass alloy, which acted as a tool holder. With this invention, they had a method to hold the diamond and manually impact the rock without shattering the brittle diamond. Hundreds of workers would excavate man-sized holes, often over 70 m (230 ft) in depth, to gain access to fresh water. Today, about 35% of petroleum wells are drilled with a polycrystalline diamond cutter (PDC) drill bit that shears the rock, and the balance is drilled with a roller cone drill bit, which primarily crushes the rock. Over 5000 km (3000 miles) of hard rock are drilled for geothermal and petroleum wells every year worldwide, primarily with the roller cone drill bit. An annual hard rock drill bit market growth rate of 7% is predicted for the next three years, producing a strong incentive to increase the efficiency of drilling, either by increasing the rate of penetration, tool life, or both. It has been shown * Fellow of ASM International **Member of ASM International
that doubling the rate of penetration in hard rock can reduce well costs by 15%. Thermally stable polycrystalline (TSP) diamond drill bits can potentially increase the rate of penetration in abrasive and medium-to-hard rock by a factor of two compared with roller cone bits. Resulting efficiency savings would produce a potential drilling product value to the mining, geothermal, and petroleum industry in the range of $200 million to $500 million per year. This is a high financial incentive to develop new hard rock drilling technologies for improving the energy economy, to reduce coal, geothermal, and petroleum energy costs. Brazing is a common technology for joining synthetic polycrystalline diamond to tungsten carbide. Assuming that the diamond and the tungsten carbide have been heated uniformly, as in vacuum brazing, finite element modeling studies have shown that thermal residual stresses in the diamond increase with increasing braze temperature. Furthermore, the lowest permissible brazing temperature should be used consistent with the strength required for the application; and the thicker the braze foil thickness, the greater is the stress relaxation. These studies also show that critical thermal residual stresses (resulting in delamination or cracking) develop with a braze filler metal thickness of less than 50 microns. Depending on the process, these joints could have low shear strengths and delamination due to thermal residual stresses. To alleviate these difficulties, we have developed two new processes — microwave brazing, combustion synthesis brazing, and combinations of these two. This article describes these techniques, and discusses their benefits and limitations. Microwave brazing Microwave brazing involves heating an assembly of tungsten carbide and the TSP diamond with a braze foil between them. This assembly is heated to a temperature high enough to melt the braze metal. As a result, a bond is developed between the TSP diamond and the tungsten carbide. This method has the following advantages over conventional methods: • Diamond is preferentially heated because it has a lower coefficient of thermal expansion than tungsten carbide. This provides the ability to match
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1186 °C
DTA, mW/mg
60 1314.6 °C 499.7 °C
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600 800 1000 Temperature °C
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Fig. 3 — Differential thermal analysis curve of a typical multilayer assembly heated at a rate of 40K/min. Two exothermic peaks and an endothermic peak are visible. The endothermic peak corresponds to the melting point of the NiTi equiatomic compound.
Fig. 1 — The Nickel-Titanium phase diagram. The phase of interest for the CS reaction is NiTi, which melts congruently at 1310°C.
Fig. 2 — Cross-sectional transmission electron micrograph and the corresponding electron diffraction pattern from the as-deposited nickel-titanium multilayer deposit. Discrete layers of nickel (dark contrast) and titanium (light contrast) are visible. The electron diffraction pattern confirms that nickel and titanium do not react in the asdeposited condition.
the thermal expansion of the dissimilar material pair. • The process can control the magnitude of thermal residual stresses. Thus, it has been possible to produce good joints without any stress cracking in the samples. Brazes with good bonding have been produced with a variety of braze filler metals, including CuSil, TiCuSil, Palcusil-25, and Wesgo Metals Palnicurom-10. • The maximum attachment shear strength can be attained with a thin braze metal foil (less than 50 microns). Combustion synthesis Combustion synthesis (CS), or self-propagating high-temperature synthesis (SHS), relies on the ability of highly exothermic reactions to be selfsustaining. In a typical CS reaction, the mixed reactant powders are pressed into a pellet of a certain green density and subsequently ignited. The exothermic reaction is initiated at the ignition temperature, and the generated heat is sufficient to carry the reaction forward to completion. Thin films
or foils could be substituted for the powder. In fact, thin films are better at reducing the ignition temperature because of their good thermal contact. Accordingly, in this investigation, we used thin films of materials deposited by the physical vapor deposition process of sputtering. The process The nickel-titanium phase diagram is shown in Fig. 1. The NiTi equiatomic compound undergoes a martensitic transformation. The martensite phase is soft, has a monoclinic structure, and exhibits the shape memory effect. In addition, the heat of reaction for the formation of NiTi is high enough for the CS reaction to take place. In our studies, individual layers of nickel and titanium of different thicknesses were deposited on different substrates (sodium chloride or formvar). These substrates could be easily dissolved to build free-standing films for further characterization. Silicon was selected as the substrate when specimens were needed for cross-sectional transmission electron microscopy (TEM). The thickness and the number of layers in each assembly were varied. Figure 2 shows a cross-sectional TEM micrograph and the corresponding electron diffraction pattern of the assembly in the as-deposited condition. Discrete layers of nickel and titanium can be observed, showing that no reaction ensued between them in the as-deposited condition. This has been confirmed both by X-ray and electron diffraction techniques. A typical differential thermal analysis (DTA) curve of the multilayer assembly, heated at a rate of 40 K/min, is shown in Fig. 3. Two exothermic peaks and an endothermic peak are visible. The first exothermic peak appears at 500°C (930°F) and the second at 1186°C (2166°F). The endothermic peak is at 1310°C (2400°F), almost exactly corresponding to the melting point of the NiTi compound. An X-ray diffraction pattern of the sample heated to 750°C (1380°F), beyond the first peak temperature, is shown in Fig. 4. Note that the major phase present in this pattern is the monoclinic NiTi phase. In addition, minor quantities of
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1200 NiTi 111
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Fig. 4 — X-ray diffraction pattern of the sample heated to 750°C shows that the major phase present is NiTi with the monoclinic structure.
other phases appear, including NiTi2 and Ni4Ti3. However, as suggested by the DTA curve shown in Fig. 3, indications are that further heating to higher temperatures such as 1200°C (2190°F), may completely transform the whole assembly to the NiTi phase.
Tungsten carbide
Fig. 5 — Transmission electron micrograph of the sample heated to 800°C exhibits an equiaxed structure suggesting that significant diffusion had occurred between the individual nickel and titanium layers to form the intermetallic compounds.
Tungsten carbide
Tungsten carbide
Ni
TiCuSil
TiCuSil
Ti
Brazing results Significant diffusion developed Ni between the discrete layers of TSP diamond TSP diamond TSP diamond nickel and titanium after the assembly was heated to higher temperatures. This led to the formation of an equiaxed structure of a b c predominantly the NiTi phase Fig. 6 — Three different brazing assemblies to explore the efficiency of combustion synthesis brazing. (a) (Fig. 5). Because the CS reaction did Conventional brazing, (b) Conventional + CS brazing, and (c) CS brazing. produce the NiTi phase, it should be possible to use this reaction heat to braze the TSP sible with other braze filler metal compositions, and diamond to tungsten carbide. This has been tried can also join other dissimilar materials. Research is under way by Technology International Inc. at the under three different conditions. First, base line data was based on placing a Department of Metallurgical and Materials EngiTiCuSil (a reactive braze filler metal) foil between neering of the Colorado School of Mines, for dethe diamond and the tungsten carbide (Fig. 6a). The veloping newer braze filler metal compositions and ■ next attempt was to deposit alternate layers of nickel improving the process efficiency. and titanium on both the diamond and tungsten carbide samples, and then place the TiCuSil foil be- For more information: Professor C. Suryanarayana, Detween them (Fig. 6b). The final variation was to coat partment of Metallurgical and Materials Engineering, the tungsten carbide and diamond samples with Colorado School of Mines, Golden, CO 80401-1887; tel: alternate layers of nickel and titanium, with no 303/273-3178; fax: 303/273-3795; e-mail: schallap@mines. edu. braze foil between them (Fig. 6c). Results showed that the heat generated in the The authors wish to express recognition for the support of this exothermic reaction between nickel and titanium project by William Gwilliam of DOE NETL and Brian Gahan is sufficient to braze the assembly. Further, since of GRI. They would also like to recognize Martin Barmatz and NiTi is soft and ductile, this compliant layer be- Nasser Budraa of NASA JPL for the development of microwave tween diamond and the tungsten carbide could brazing process and Isaac Dahan of the Colorado School of easily accommodate the thermal residual stresses Mines for help in electron microscopy work. during cooling from the braze temperature. How useful did you find the information For diamond brazing applications, the process presented in this article? must be controlled so that the brazing temperature Very useful, Circle 288 does not exceed 1200°C (2190°F), the temperature Of general interest, Circle 289 above which diamond converts to graphite. Not useful, Circle 290 The combustion synthesis brazing method is posADVANCED MATERIALS & PROCESSES/MARCH 2001
