Microstructural changes can be induced in almost all engineering materials to alter their mechanical properties. This is usually achieved through thermal treatments involving heating and cooling under controlled conditions. The treatment temperatures and processing conditions vary according to the nature and composition of the material. Combinations of mechanical and thermal treatments are sometimes used (i.e., thermomechanical treatments) to produce properties that cannot be obtained in any other way. In some cases, different properties are desired in the bulk than in the surface of a part. In these instances, specialized thermal treatments involving surface diffusion or surface (differential) heating are used.
This chapter discusses the main categories of structure-change processes used for metals, ceramics, and polymeric materials and for coating and laser processes that produce desirable structural change.
Metals And Alloys
Iron-base alloys, particularly the carbon and low-alloy steels, are among the most widely used structural materials in industry. Structure change can be achieved easily in iron-carbon alloys using straightforward thermal treatments that rely on the change in solubility for carbon and alloying elements that accompany iron's allotropic transformations. As expected, mechanical properties vary with the structure. Furnace-cooled (i.e., slow cooling rate) materials exhibit low strengths and high ductilities, since they are fully annealed, while rapidly cooled materials (water-, oil-, or air-quenched, depending on the alloy content)
exhibit higher strengths and lower ductilities. Volume change accompanying the austenite-martensite transformation can lead to high residual stresses and substantial distortion, along with a risk of part cracking, due to differential cooling in parts with large changes in cross section. Hence, hardened parts are usually tempered to relieve the stresses and to stabilize the part dimensions.
Structure-change processes in nonferrous (i.e., aluminum) alloys are widely used in aerospace construction and are finding increased use in the automotive industry to reduce vehicle weight. These materials depend on change in solubility of alloying elements with temperature. However, in the absence of allotropic transformation in the base material, changes in mechanical properties depend on solution treatment followed by precipitation aging. The final structure and properties in these dispersion-hardened materials depend on the times and temperatures used for the precipitation treatment. The upper temperature limit for the use of these alloys is determined by the propensity of the precipitates to coalesce and coarsen.1
For example, superalloys used to build the hot-section components of turbine engines rely on a structure comprising a nickel matrix with a fine dispersion of highly stable gamma prime precipitates (i.e., Ni3Al) that possess a good lattice match with the matrix. Such a microstructure retains its strength even at relatively high temperatures, hence the name ''superalloy.''
Ceramic And Glassy Materials
Principles of structure change in ceramics are similar to those for metallic materials. However, the thermal treatment practice is handicapped by the low thermal diffusivity inherent to ceramics. Also, slow heating and cooling rates are used to minimize risk of cracking by thermal stresses. This problem is aggravated by the higher thermal treatment temperatures necessary for structure change in ceramics.
Structure change in nominally amorphous ceramic materials uses thermal and thermomechanical treatments comparable to those for metals and alloys (Weidmann et al., 1990). Glass-ceramic oven tops, for example, are mechanically processed and thermally treated to create a final microstructure with a very low thermal coefficient of expansion. Exceptional thermal shock resistance is obtained through control of microstructure.
Ceramics pose problems in structure change because of the directionality of their molecular bonding, high inherent hardness, slow achievement of
equilibration, limited number of slip systems with resulting very low ductility, and the presence of hard and soft glide planes. Despite these limitations, thermally treatable, tough ceramics such as partially stabilized zirconia have been developed and are finding increasing use as structural ceramics.
Nominally amorphous polymeric materials are normally processed by melt-to-solid transformations. However, extraordinary strength gains can be obtained in some polymers (e.g., polyethylene) by controlled deformation in the processing of fibers. Precise strain, strain rates, and thermal treatments are necessary.
Structure-change processing to obtain high strengths in common polymers is not widely used. Injection-molded tubular polyethylene terephthalate preforms are stretched and blow molded when the material is elastomeric. Processing conditions are selected and used to prevent any structure change between injection molding and heating for blow molding. Process temperature and time are adjusted to suppress spherulite formation and to promote partial crystallization. Final strengths obtained in the part (e.g., plastic beverage containers) are comparable to those of aluminum alloys.
Many coating processes have been developed to deposit microthin films that allow close control of coating thickness and composition not possible with traditional surface treatment processes (e.g., surface hardening, electroplating, and plasma spraying). These processes also allow well-bonded thin films to be deposited at relatively low temperatures. Examples of these thin-film deposition technologies currently in commercial use include halide metallurgy (i.e., chemical vapor deposition [CVD] processes) and electrically assisted vacuum coating (i.e., physical vapor deposition [PVD] processes). In CVD processes, halide compounds such as TiCl4 with hydrogen are used to deposit thin films on hot substrates. The coating temperature is typically above 750 °C. Process streams and operating conditions are chosen to obtain viable coating rates and to maximize deposition efficiency. Specially developed CVD phase diagrams and computation packages are used. Kinetic variables such as the total flow rate, coating system geometry, substrate chemistry, and surface finish play a role in determining the nature and quality of films deposited (Bhat, 1989). Cobalt and other metal-bonded carbide tool inserts are now routinely CVD-coated with
compounds such as TiC, TiN, and A1203, singly or in combination. High temperatures used in CVD make post-coat thermal treatments necessary. for steel tools and parts. Coating internal stresses and thermal stress-induced cracking of the deposited films are common. CVD for hard coating of steel products is therefore not common. Light alloy parts are not coated with CVD processes.
Since hard coating of ferrous components and most mechanical parts requires lower deposition temperatures, PVD processes are used. To date, a variety of film-deposition techniques have been developed to produce the needed coatings:
- activated reactive evaporation (Bunshah and Raghuram, 1972);
- ion plating;
- direct current, radio frequency and magnetron sputtering (Bunshah, 1982);
- hollow cathode discharge (Komiya and Tsuruika, 1976); and
- are coating (Ramalingam et al., 1987).
The structure, properties, and adhesion of the films deposited with PVD processes depend on coating-source characteristics, the substrate temperature, bias applied, and the ambient pressure (Thornton, 1974). By admitting reactive gases during film deposition, coatings of compounds are obtained. Control of compound stoichiometry requires coupling of the gas admission rate with the coating-flux generation rate.
All the thin-film deposition technologies mentioned above, as well as the more recently developed microwave-assisted plasma deposition processes, are in commercial use to support integrated circuit fabrication. In these applications, deposition occurs on very flat surfaces in relatively thin films (less than 1 millimeter in thickness).
Lasers can be used to alter the microstructure and properties of metals, polymers, ceramics, and glasses. Lasers can change the surface properties of finished parts without affecting the parts' inherent bulk properties. This capability can be used to clad material onto a worn surface, thereby greatly extending a part's useful life, or to repair high-value components without causing extraneous damage. Although lasers do not require any special processing environment, such as a vacuum, environmental control can be utilized when the material being processed requires it.
Laser heat treatment involves traversing the laser beam over a large surface area of a metal workpiece. Through rapid heating by the laser beam and subsequent quenching, the microstructure of a layer of material near the surface can be modified without affecting the bulk of the workpiece. Laser heat treatment of alloy steels can significantly increase their strength, toughness, sliding wear resistance, and abrasive wear resistance. The microstructure formed near the surface is essentially dislocated packets of martensite surrounded by retained austenitic films. Laser heat treatment can also be used to relieve residual stresses caused by mechanical processes through annealing. One example is selective stress relieving of glass components using a carbondioxide laser.
Laser heat treatment is currently limited to specialized operations, mainly due to its low processing speeds for continuous production. However, as the energy densities of new lasers increase, laser heat treatment may become economically viable for the large-scale continuous processing of metals, since much higher scan rates and beam velocities could be attained. Another development may be the use of an array of diode lasers as a direct heat source. This approach has a number of advantages. First, the diode array would allow area coverage of the workpiece, resulting in a uniform heat treatment. Second, the diode array is stationary, providing better reliability than a moving laser head. Finally, diode arrays can be placed next to both the top and bottom surfaces of the workpiece, allowing simultaneous heat treatment of both sides (Warner and Sheng, 1993).
Lasers can be used for surface modification processes (i.e., laser melting, cladding, alloying, and peening) in which the microstructure of a workpiece surface is modified through laser melting and rapid solidification of a thin layer of material (Ortiz et al., 1990). This melting and solidification is often accompanied by the introduction of powder elements or a predeposited layer of new material to combine with the base material. These processes are influenced by factors such as the laser power and power density, size and shape of the beam profile, scan velocity, and chemistry and metallurgy of the substrate.
Laser melting involves rapid heating and phase change of a small surface layer of a substrate through beam impingement and subsequent rapid quenching. To melt a material, a laser power density of 105-107 watts/cm2 is usually required. The melting and solidification rates are so rapid that most elements go into solution with little opportunity to precipitate back to the grain boundaries. By controlling parameters (e.g., laser power, translation speed, peak power, pulse rate, etc.), the substrate melt depth can be controlled to range from a few micrometers to a millimeter.
Laser surface alloying is a process in which the surface of an alloy is melted to a desired depth using a continuous-wave or pulsed laser beam with the simultaneous addition of powdered alloying elements. The alloying element can
be either preplaced or added as a powder stream during processing. The combination of convection and diffusion redistributes the alloying elements uniformly throughout the molten pool. Laser surface alloying can result in the synthesis of nonequilibrium metallic phases that could lead to a wide variety of microstructures, including extended solid solution and amorphous phases. Alloying depths can range from less than 1 micrometer to over 1 millimeter and be free of porosity or cracks. An advantage of laser surface alloying over conventional methods is the small heat-affected zone created, which allows the finished part to retain the bulk characteristics of the base material. The metallurgical bond also helps to provide a high degree of adhesion between the bulk material and the laser alloyed region.
Laser cladding is similar to laser surface alloying except that the powder constituents introduced into the melt pool are also melted by the laser beam. The main objective in laser cladding is to overlay a surface or substrate with another material that has a different chemistry by melting a thin interfacial layer to produce a metallurgical bond with minimum dilution of the clad layer. Typical laser power densities range from 105 to 106 watts/cm2. The most significant advantages are production of novel alloys, minimized clad dilution, reduced alloy material loss, reduced machining, and reduced distortion. Due to the laser's rapid melting and quenching capabilities, fine microstructure, increased solid solubility of alloying elements, and nonequilibrium crystalline and amorphous phases are possible. Laser cladding is especially useful for creating wear-resistant surfaces when the component material is not conducive to transformation hardening due to the high process speeds and minimization of heat distortion. A potential problem with parts subjected to localized surface melting is that high residual tensile stresses are developed unless a phase change, with volume expansion, takes place.
Shot peening is a well established process that typically uses small particles to impart localized plastic deformation. Properly done, it results in a residual state of compressive stress at the surface. An innovative approach uses a pulsed laser beam to perform peening (Ortiz et al., 1990). The beam impinges on a layer applied to the substrate; normally a paint layer (with impingement occurring underneath a water film) or copper foil are used. The vaporization of the paint layer or foil creates a plasma plume. The expanding plume creates an intense pressure wave that is directed onto the surface; it results in a compressive stress region that extends approximately 0.5 mm in depth from the workpiece surface. This compares favorably with the compressive stress depths of 0.2-0.3 mm that are achievable through shot peening. However, with a Q-switched laser operating at pulse lengths of 10-30 nanoseconds and 0.1 joules/pulse, only 1 mm2 of surface area can be treated per pulse. This translates into a relatively slow processing speed of 10 inches2/min.
Structure change in metallic alloys, ceramics, and polymers has a sound theoretical basis, and the structure-change processes in use are well established and understood. The ease with which structure change can be effected in metallic alloys has led to the widely used industrial practice of fabricating metallic parts in their soft condition, followed by structure-change processing either as a final or penultimate treatment to obtain the design properties. A major deficiency in structure-change processing for discrete parts is its frequent dependence on batch processing for thermal treatment, since undesirable storage queues are typically required. Two recent developments offer a means of overcoming this problem: solid-state, high-frequency power sources and continuous-mode, pulsed, and solid-state lasers. Both developments permit rapid, in-line structure-change processing, which eliminates the need for batch processing, and hence in-plant storage is minimizied.
The technology base related to thermal treatment processes (e.g., annealing, normalizing, spherodizing, solution treatment, aging, hardening, tempering, homogenizing) applies to all types of materials. Some high-priority research needs are described below for the critical areas of microstructure control, product/process control, and process development.
Models applicable to all materials of interest are needed to simulate the evolution of a microstructure as a function of thermal history and process conditions (furnace and ambient) to predict final structure distributions and product properties. Extension of these simulation models into process control can be used for real-time process optimization. Specifically, the microstructure evolution models should be capable of incorporating:
- dissolution and precipitation processes;
- physics of structure change effects during quenching;
- models of kinetics of recovery, recrystallization, and grain growth; and
- models of microstructure change in metastable materials (such as rapidly solidified materials) during thermomechanical processing.
Rapid thermal treatments including induction, laser, or directed-beam heating are increasingly important in structure-change processing. Extension of microstructure response models to include these rapid thermal processes would
enable identification of the process condition windows that are required to predict product properties and process economies.
Product quality of thermally treated parts is usually expressed in terms of mechanical properties (e.g., strength, hardness, toughness) and geometric tolerances. Since typical problems encountered during thermal processing are part distortion and cracking, a better understanding of thermal and mechanical mechanisms is required to allow prediction of conditions that cause shape change (distortion) and cracking during thermal processing.
Improved thermal treatment processes for structure change of metal alloys require robust sensors to monitor furnace atmosphere and conditions (temperature, partial pressures, flow patterns), as well as product state (i.e., microstructure), during processing. In-process measurement of quenchant characteristics is also needed. In-process measurements will assist quantifying process improvement, enable further process refinement, aid in planning preventive maintenance, and improve equipment utilization. Development of in-process sensors will also be useful to minimize the environmental impact of thermal treatment processes (e.g., decrease emissions) and to enhance process energy efficiency.
Structure-change processes for product property and quality enhancement require further development for ceramic materials and polymers. (Notable exceptions are the pyroceramic materials and polyethylen tenephthalate, which are well developed.) Improving structure-change processes for these materials will also require R&D in sensors and process characterization.
Although thin-film coating treatments are finding wide use in the tooling industry,2 use of such surface treatments in the mechanical component industry is less common. Processes that deposit thin wear-resistant coatings could be extended to many mechanical component applications in which wear and durability are key concerns. Such technology applied to automotive applications, for example, could facilitate the increased use of nonferrous lightweight alloys (e.g., aluminum, magnesium, and titanium alloys) and thus indirectly lead to
substantial improvements in manufacturing efficiencies and fuel economy.3 Mechanical applications require coating three-dimensional parts with complex materials possessing precise stoichiometry. Films deposited must also have exceptional film-to-substrate adhesion to withstand high contact stresses and severe localized power dissipation due to frictional contact.
Despite use of thin-film deposition technology to support the cutting tool industry, a science base to support the coating process needs of mechanical industry has not yet emerged. The following research is needed:
- Process characterization. High-priority issues include the determination of coating flux density, distribution of the flux emerging from the coating source, and mean energy of the flux, in order to assure reproducible stoichiometry in films, high-film deposition rates, microstructure control (i.e., control of grain size, morphology, texture, and orientation and control of film porosity), and coating uniformity. Understanding the film stress evolution and mechanism of good film-to-substrate adhesion is vital in order to take advantage of thin films to reduce surface distress, especially in nonferrous materials.
- Coating flux generation. While flux generation processes are well understood for thermal deposition and sputtering processes, basic processes involved in emerging coating processes (e.g., cathodic arc deposition and anodic arc-based processes) require further study.
- Process modeling. Understanding process characteristics and the mechanisms involved in the generation of coating flux will enable process models to simulate film deposition so that coating processes can be designed. Process development now depends on trial and error to identify acceptable processing conditions. Process modeling will allow control of nucleation and growth in films, especially as the models relate to structure development, film-to-substrate adhesion, and generation of film stresses.
- Scale-up. Laboratory-scale PVD processes are difficult to scale-up to production-level systems. This difficulty is partly due to the three-dimensional shape of mechanical products and the relatively large spatial volumes over which stable and metastable materials have to be synthesized homogeneously in order to coat large pans uniformly. Process scaling parameters must be identified and understood.
- Sensors. Concurrent synthesis and reliable deposition of high-performance materials require development of sensors to monitor changes in critical process variables. Sensors that only measure vacuum and flow-rate cannot
Recent Department of Energy workshop recommendations support and emphasize this suggestion (Courtright, 1993).
- provide an adequate basis to monitor synthesis of film materials and their deposition on substrates, so that well-bonded films with minimal film/substrate stresses are consistently produced.
- General process understanding. Studies are required for better understanding of processes for the deposition of ultra-high-hardness materials including diamond and cubic boron nitride, deposition of multilayer films such as microlaminates, deposition of synthetic microstructures (patterned, two-phase coatings), and the deposition of alloyed hard materials.
The recommendations presented in Chapter 4 regarding the development of new types of lasers, modeling, sensors, and control apply also to the use of lasers for structure change. (At the present time, the use of lasers for surface treatment is a very small fraction of their use for cutting and welding. The reason lies in the ability of a laser to generate an extremely high energy density at its focal point that can readily be used to cut and weld thick plates. However, for surface treatment, the beam must be defocused to avoid vaporizing material. As a result, the laser loses some of its unique advantages.)
Progress in laser surface treatment must include the development of alloys optimized for laser treatment. (Many of the alloys used in laser treatment were originally developed for welding.) Material characterization and modeling will be required to fully understand the effect of extreme heating and cooling rates on the nonequilibrium microstructures that may result.
Bhat, D.G. 1989. Chemical Vapor Deposition. Surface Modification Technologies, T.S. Sudarshan, ed. New York: Marcell Dekker.
Bunshah, R.F., and A.C. Raghuram. 1972. Activated reactive evaporation process for high-rate deposition of compounds. Journal of Vacuum Science Technology 9(6): 1385.
Bunshah, R.F., ed. 1982. Deposition Technologies for Films and Coatings. Park Ridge, New Jersey: Noyes Publishing.
Courtright, E.L. 1993. Workshop on Coating Needs in the Auto Industry. Prepared for the Office of Transportation Materials, Division of Conservation and Renewable Energy, U.S. Department of Energy. Contract no. DE-AC06-76RLO. Washington, D.C.: U.S. Government Printing Office.
Komiya, S., and K. Tsuruoka. 1976. Physical vapor-deposition of thick Cr and its carbide and nitride films by hollo-cathode discharge. Journal of Vacuum Science Technology 13(1):520-524.
Ortiz, A., C. Penny, M. Jones, and C. Erikson. 1990. Laser Peening Systems and Method. U.S. Patent No. 4,937,421.
Ramalingam, S., C.B. Qi, and K. Kim. 1987. Controlled Vacuum Arc Material Deposition Method and Apparatus. U.S. Patent No. 4,673,477.
Thornton, J.A. 1974. Influence of apparatus geometry and deposition conditions on structure and topography of thick sputtered coatings. Journal of Vacuum Science Technology 11(4):666-670.
Warner, B., and P. Sheng. 1993. Special report: new laser technology, better lasers create applications and improve existing uses. Industrial Laser Review 8(7):9-13.
Weidmann, G., P. Lewis, and N. Reid, eds. 1990. Structural Materials. London: Butterworth Scientific Ltd.