POTENTIAL OF COATED POWDERS FOR POWDER INJECTION MOLDING

Animesh Bose, Asit Biswas, and A.J. Sherman

Powdermet, Inc., Sun Valley, California 91352

1. ABSTRACT

Using microengineered powders for processing advanced materials is gradually gaining acceptance in several industrial sectors. Microengineered coated powders provide materials engineers with an exceptionally powerful tool for processing high performance materials. The powder injection molding industry, however, has yet to take advantage of these microengineered powders’ benefits. This paper will discuss the concept of microencapsulated powders and their potential for use in the PIM industry.

Powder injection molding (PIM) is a unique processing technique capable of producing complex shapes from numerous high performance materials [1,2]. This relatively new technology has come of age over the last ten to fifteen years. The process offers the shaping advantages of traditional plastic injection molding, and expands applications to numerous advanced materials such as metals, alloys, ceramics, cermets, intermetallic compounds, and composites [1,4].

In its basic form, the PIM process consists of mixing a small amount of organic binder material (typically polymers, waxes, oils, etc.) with the desired inorganic powder (metal, alloy, intermetallic compounds, composites, etc.). The powder and organic binder mixture is cooled and granulated or pelletized, resulting in small, discreet pieces that can be fed into an injection molding machine hopper. This is known as the PIM feedstock. The feedstock is introduced into the injection molding machine’s hopper, where it is re-melted in the barrel and pushed into an oversized die cavity. This molded shape (commonly referred to as the green part) is usually an oversized replica of the final product.

In special cases, the as-molded shape itself may be the final part (then the green part becomes the actual part). Generally, the green part is subjected to` a step known as de-binding, where the part’s organic component is removed. De-binding is used to gradually and completely remove the organic phase, without leaving any residual contamination. After the binder is removed the part is subjected to a thermal treatment known as sintering, which results in a desired level of part densification. It is entirely possible that pressureless sintering itself will not result in desired part density; in such cases, post-sinter densification processes can be used. Post-sinter heat treatments and additional mechanical coining steps are also used to attain desired properties. This process is well suited to producing relatively small, moderate to high volume parts (golf ball size is the conventional PIM process’ upper limit). Thus, the PIM process provides designers and engineers with a powerful technique capable of shaping materials such as plastics, while not confining them to plain organic polymers (thermoplastic or thermosetting types).

While the above paragraphs briefly describe the generic PIM process it should be understood that the process has numerous variations, although the basic steps are typically the same. Variations result from different combinations of powders, organic binders, mixing and molding techniques, numerous de-binding routes (again that depend on initial binder choice), and widely varied sintering practices. For example, a number of interesting variations in PIM stainless steel processing have been used, with widely varying de-binding practices ranging from wicking and solvent de-binding to a pure thermal de-binding process. Therefore, many different PIM processing routes may be used to create the same product.

PIM has tremendous advantages over other processes in manufacturing complex shaped parts and parts made from expensive or difficult to machine materials. High temperature materials such as ceramics or refractory metals and alloys are often powder injection molded to yield complex shaped parts. Advanced composite materials generally are expensive and difficult to machine; therefore machining them into complex shapes is not considered a practical manufacturing method. Powder injection molding is one of the only shaping processes available for fabricating complex shaped components from such materials. Some advanced composite materials ideally suited for the PIM process are AlN-Al composites, W-Cu and Mo-Cu, tungsten heavy alloys, and WC-Co or Tic-Fe/Ni-based hardmetals. However, availability of suitable powders presents a problem when processing these materials using the PIM route.

A discussion of ideal PIM powder characteristics is merited, as this process has several special requirements. The ideal PIM powder should not react with the organic binder, should possess either a narrow or wide size distribution, have a relatively spherical nature so that high solid loading into the organic binder is possible, and should have a relatively fine size range (between 1 to 10 mm representing a very good processing range). The powder particle size should have some surface roughness or irregularity to provide high interparticle friction, which allows better shape retention during the de-binding stage. Other important properties that have a fundamental impact on feedstock homogeneity are using relatively close powder particle sizes to avoid any separation during mixing, and uniform distribution of two different powders for proper homogenization and/or distribution of one phase in the other. The problem of powder separation is accentuated when there are two different powders of varying sizes (where the smaller powders do not fit into the interstitial gaps and increase packing density), when the powders have very wide density differences, and when the volume fraction of one powder is small compared to that of the other. The above factors would increase inhomogenities within a feedstock and would result in impaired properties.

Microencapsulated powders provide engineers with a new powder source that could be extremely suitable for the PIM process. Microencapsulated powder is a particulate material in which each powder particle is coated with a layer of a different material, if properly processed. For example in W-10 Cu-based composite powder, the microencapsulated powder approach would result in applying a thin copper coating layer onto tungsten powders. Compared with the conventional approach where copper powder is mixed with tungsten powder and the powders remain separate until the composite is sintered, the microencapsulated powder approach provides a composite powder wherein the copper no longer has to be mixed separately with the tungsten. Using microencapsulated powders provides advantages in processing such advanced composite materials using PIM. Some conceptual uses for microencapsulated powders in the PIM industry will be discussed.

The process of powder injection molding has come far in terms of injection molded materials. One reason for the relatively slower-than-potential growth rate of this industry is non-availability of suitable powders. Large powder producers have been reluctant to enter the PIM market, as PIM powders are used in small quantities and have diverse and often conflicting property requirements. This is especially true in the case of new, untried materials. Lack of proper powders has, to some extent, impeded the technology’s rapid spread into such industries as electronics, chemicals, and aerospace. This powder availability problem is especially critical with advanced materials such as W-Cu, Mo-Cu, AlN-Al, hardmetals and cermets, refractory metals and alloys, and composites based on refractory metals and alloys.

Three popular powder production approaches are used in the PIM industry (especially in the metal injection molding industry) to process advanced composites and alloys:

1) Pre-alloyed powders with the desired characteristics. Examples of such powders are 316L stainless steel, 17-4 precipitation hardened stainless steels, several tool grade steels, etc.

2) Individual powder constituents. These generally are used in elemental form mixed in the correct proportion and combined with an organic binder. Examples of such powders are W-Cu, Mo-Cu, W-Ni-Fe, WC-Co, TiC-Fe/Ni, Fe-Ni, and Fe-Si.

3) A blend of mechanically milled mixed powders from different components. Prime examples of such materials are WC-Co and some high Ni alloys containing some Fe.

The above processing routes have problems, which become acute in powders where the different constituents are either blended or mechanically milled. Pre-alloyed powders, which currently represent a large share of the metal injection molding (MIM) market, usually are produced by two atomizing routes: gas atomization, and water atomization. Powders produced using these methods have advantages and disadvantages in terms of their suitability for use in PIM.

Both industry segments are trying to defeat the powders’ shortcomings through various mechanical means. Gas atomized powders generally are spherical in nature and must be classified to obtain the desirable powder size range, significantly increasing powder costs. The powder’s spherical nature allows higher solids loading, easier molding, and less dimensional shrinkage. However, these powders have low interparticle friction and therefore are prone to distortion and slumping during debinding events. Coarse spherical powders (greater than 20m m) are usually the poorest, as they are extremely prone to slumping and distortion. Water atomized powders have a relatively irregular nature and their use results in lower solids loading and hence greater shrinkage and increased feedstock viscosity. This results in difficulty during molding. The problem is accentuated when very fine powder is used. Yet, this powder has high interparticle friction and generally does not undergo slumping when the binder is removed.

Therefore, it becomes obvious that both powder production methods have positive and negative features regarding their use in the PIM industry. Both manufacturers are taking steps to decrease the negative aspects of their particular powders, and it is interesting to note that both types of powder manufacturers have resorted to some mechanical processing to achieve the desired goals. Gas atomized powder producers have been trying to modify powder shape to make the powders less spherical. This is being done in an attempt to increase the powders’ interparticle friction. Water atomized powder manufacturers are attempting to move in the opposite direction, with the goal of obtaining more spheroidized powders providing higher solids loading and lower feedstock viscosity.

Mixed or milled powders of two or more different materials can have some of the above-mentioned problems, depending on the starting powders. This is further complicated because it becomes necessary to attain some degree of homogeneity between the different materials. Very low volume fraction in one of the phases, large difference in density between the materials being mixed, and improper size difference between the powders being mixed can all result in feedstock problems. Also, any major powder segregation during feedstock production can eventually lead to problems during sintering, where improper powder distribution can lead to poor densification and homogenization. One method the industry adopted to solve this problem is milling the different materials together; however milling results in powders with an angular nature and that have poor packing characteristics. Milling often also results in impurities being incorporated that are generally detrimental in nature. However, the lack of alternate commercial powder production routes for these materials results in the PIM industry frequently using the milling method.

Using microencapsulated powders, conceptually, provides a new pool of materials for PIM processing advanced alloys and composite materials. The idea is relatively simple: instead of trying to blend or mill dissimilar materials together, powder microencapsulation coats the minor phase on top of individual major phase particles. In this case, each individual particle has the major phase encapsulated within the minor phase. The coated powder approach provides numerous advantages, especially from the point of view of feedstock homogeneity and homogenization during sintering. The following section will discuss the potential of this approach for PIM processing advanced materials.

Microencapsulated powders can benefit several popular alloys used in the PIM process such as Fe-2Ni, Fe-7Ni, Fe-3Si, Fe-P, and 316L. The nickel steels are one of the most popular alloy classes used in the PIM industry. They are extensively employed as structural components in the as-sintered, carburized, or carbonitrided form in a number of industries including household goods, gun components, and business equipment. Industry generally uses blended elemental nickel and iron powder for the feedstock, and milling usually is avoided unless the nickel content is very high.

As stated above, the material is normally suitable for use as structural components, except for in cases where there are compelling reasons for almost eliminating residual austenite phase in the microstructure. Using nickel (an austenite stabilizer) is generally not a problem, as the high temperature treatment used to sinter these alloys also results in nickel homogenization. However, when using higher nickel containing steels the chances of incomplete homogenization are increased, which in turn increases the chance of retained austenite within the microstructure. During low temperature exposure, the retained austenite could undergo transformation and create stresses within the structure, resulting in premature failure of the part in service.

Retained austenite in a martensitic microstructure is especially undesirable in carbonitrided parts used in close-fitting assemblies. In these cases, the retained austenite not only lowers hardness, but also could actually bind or "seize up" during operation, due to volume increase during transformation. It is possible to reduce retained austenite by using sub-zero treatments, but this could lead to the possibility of microcracks on the case structure, especially if the grain size is quite coarse.

It is therefore of great help for the nickel in PIM nickel steel to be well homogenized. The conventional route of blending iron and nickel elemental powders is apt to produce some areas where the nickel has not been completely homogenized. This could lead to the chance for formation of retained austenite after carbonitriding. If, however, nickel is deposited as a coated layer on the iron powder surface, then uniform distribution of nickel on each powder particle will significantly reduce diffusion distances and therefore homogenization time.

Using these powders in the PIM industry can provide significant advantages. Yet using these powders (which would add to cost associated with the microencapsulation process) can only be justified in cases where parts require very low retained austenite within the microstructure. It is expected that the coated powders can solve some of the above-mentioned problems in the PIM industry.

Prealloyed powders represent the other form of PIM alloy that could benefit from the microencapsulated powder approach. In this case, the conventional powder production approach would be either water or gas atomization. The material’s constituents are melted together and then atomized, such that almost every powder particle has the same overall powder chemistry. A popular alloy used in the PIM industry is stainless steel. Among the stainless steels used by the PIM industry, austenitic stainless steel 316L is one of the most popular. It has been found that adding boron to austenitic stainless steels can result in material sintering to high densities using a lower sintering temperature. It is possible to obtain an excellent combination of high mechanical properties and good corrosion resistance in austenitic stainless steel with a boron content of around 0.8 wt% [4]. It has also been concluded that boron is a potent additive in tool steels, where boron was found to be more potent than graphite [5]. Boron can be added either in the elemental form or as a compound. Boron addition to iron-based materials would result in a Fe2B intermetallic formation, which in turn forms an iron-rich eutectic at about 1175° C [6].

The liquid phase, which forms at the particle boundaries, has a high solubility for iron and allows faster diffusion of the iron atoms, promoting rapid part densification. Iron-based alloys sintered with boron additions exhibit characteristics of a classic liquid phase sintering mechanism [7]. It can be estimated that using 0.4 and 0.6 wt% boron addition will result in 9 and 16 V% liquid phase formations, which is quite significant.

It is clear that using boron as an additive in iron-based alloys has tremendous potential. One problem is the small amount of boron required for high densification to occur. In the case of boron addition in the particulate form, it is rather difficult to obtain uniform boron distribution. Conceptually, if a small amount of boron can be coated onto high value added iron-based alloy powder (such as austenitic stainless steels), the uniform segregation of boron at particle boundaries will result in rapid material densification. In such a case it is possible to use coarser and less expensive water atomized 316L stainless steel powder and still obtain the desired high density in PIM processed samples. What needs to be determined is whether boron distribution at the particle surface results in a continuous network of precipitates being formed along the grain boundaries, thereby degrading mechanical properties. Should this occur, it might be possibly overcome by partially substituting the boron coated 316L powder with the uncoated 316L powder.

With magnetic materials it is also conceivable that microencapsulated powders could impart property advantages by providing rapid homogenization with coated materials like iron-silicon alloys. If phosphorous coating is possible, the coating must be overlain with an iron coating, to prevent the phosphorus from burning. Similarly, it may be necessary to deposit a second overlay coating onto silicon in iron-silicon alloys.

Major applications for microencapsulated powders in the PIM industry are expected to be in the composite materials area. Some ideal composite materials that show tremendous potential for the microencapsulated powder approach are W-Cu, Mo-Cu, and tungsten heavy alloys. The W-Cu system elements have very limited solubility in each other both in the liquid and the solid phases, while the Mo-Cu system also exhibits extremely low solubility. These composites are extremely useful as thermal management materials. (Demand in the electronics and computer industries has been rapidly growing.) A major advantage of metal in injection molded housings (for thermal management) is the ability to produce complex shapes in a single piece with very little machining [8]. Another interesting area where these coated powders can be used is in the AlN-Al composite system -- an emerging thermal management material. Here it will be possible to deposit a thin aluminum coating onto AlN, which has a low thermal expansion coefficient and high thermal conductivity. The amount of aluminum used can be small, as it will be possible to produce a uniform layer of liquid on the ceramic surface. This system’s advantage over the W-Cu or Mo-Cu type systems would be a tremendous parts weight reduction. This will be critical in space related applications.

Another composite being extensively investigated in both commercial and ordnance sectors is tungsten heavy alloy. This material has a unique combination of high density, high strength and elongation, good toughness, and corrosion resistance. These materials are extensively used as kinetic energy penetrators in the ordnance area. Commercial sector applications for this material include counterbalances, radiation shields, golf clubs, and some medical devices. Tungsten heavy alloy is a two phase composite generally consisting of pure tungsten grains that are dispersed in a matrix alloy consisting of nickel-iron, which has taken some amount of tungsten into solution. The conventional manufacturing process for these materials is: blending tungsten, nickel, and iron elemental powders in the desired ratios; green consolidation of the blended powders by either axial compaction or cold isostatic pressing; followed by pre-sintering and liquid phase sintering to attain close to full density. The popular heavy alloys contain between 90 to 95 wt% tungsten. Lower amounts of tungsten have been used in special cases.

One problem with liquid phase sintering is that it generally results in some degree of slumping. Shape retention is a major consideration with PIM parts, as the parts are complex shaped. With tungsten contents at around 90 wt%, liquid formed during liquid phase sintering can and often does result in slumping to some degree. This creates difficulties in making complex shaped PIM parts from tungsten heavy alloys. Complex sintering cycles often must be designed, relying on extensive amounts of solid state sintering followed by rapid and short excursion into the liquid phase region.

Arguably, for PIM tungsten heavy alloys shape retention problems can be significantly reduced by using a totally solid state sintering process. Unfortunately, solid state sintered tungsten heavy alloys are quite brittle. This brittle behavior can be attributed to the microstructure’s high contiguity, with the W-W contacts being the structure’s weakest link. However, in coated powders where the tungsten particles are encapsulated in an iron and nickel coating layer, high contiguity is significantly reduced. The sintering process is also activated, thereby making it possible to conduct solid state sintering and yet achieve significant properties in these materials. An added advantage of the coated powder approach is that almost all particles have a similar density range, whereas in the conventional approach there is a large difference in density between nickel/iron and tungsten powder particles. This could lead to some degree of segregation.

It has been demonstrated that it is possible to get good solid state sintered properties in tungsten heavy alloys. Earlier work also has shown that high strength and high hardness heavy alloys can be produced by using refractory material additives such as Mo, Ta, and Re, with rhenium proving to be one of the most potent additives [9]. It is also possible first to coat the heavy alloys with a small amount of molybdenum or rhenium, followed by a nickel and iron coating, to determine if improved sintered properties can be obtained in these alloys. These coated powders would be beneficial for tungsten heavy alloy parts PIM processing.

The chemical vapor deposition (CVD) process used for the microencapsulation process has the ability to deposit almost any material on top of almost any other particulate material. Interestingly, this process produces a well-coated sample with the appearance of a nodular surface (much like a golf ball). When the coating is applied on an irregular powder, very thin layers generally follow the contour of the initial powder surface. However, as the coating thickness increases, there is a tendency for the total powder particle to become slightly spheroidized in nature with the applied coating. This slight spheroidization can provide higher solids loading and lower feedstock viscosity.

If, on the other hand, a thick coating is applied onto relatively smooth and satellite-free spherical powders, the final surface attains a golf ball type appearance. In other words, though the powder’s general spherical appearance is maintained, the powder’s surface roughness is significantly increased. This form of surface roughness can increase interparticle friction, without decreasing solid loading to any significant extent. Therefore, the microencapsulation process could prove beneficial for both irregular and spherical powders. The increased cost of coated powders could be offset by the ability of this process to allow coarser spherical powders to be used. These powders normally could not be used due to their low interparticle friction. Using coarser powders can significantly reduce materials cost.

The coated powder approach can bring significant advantages to the PIM process, per its ability to produce powders that have practically no segregation between constituents. These coated powders can be manipulated to allow activated sintering, such that lower sintering temperatures may be used. Through the powder coating approach it is also possible to obtain homogeneous distribution of two phases, where one of the constituents (even in the liquid phase) has extremely low solubility and wettability. The advantages of such coated powders have not yet been utilized by the PIM industry.

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2. Mutsuddy, Geebhas C., and Ford, Renee G., "Ceramic Injection Molding," Chapman Hall, London, UK (1995).

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4. Tandon, R. and German, R.M., "Sintering and Mechanical Properties of a Boron-Doped Austenitic Stainless Steel," International Journal of Powder Metallurgy, 34 [1] 40-49 (1998).

5. Kulkarni, K.M., Ashurst, A., and Svilar, M., "Role of Additives in Full Dense Sintering of Tool Steel," Modern Developments in Powder Metallurgy, Vol. 13, H.H. Hausner, H.W. Antes, and G.D. Smith (eds.), Metal Powder Industries Federation, Princeton, NJ, pp. 93-120 (1980).

6. Madan, D.S., "Enhanced Sintering and Property Improvement in Ferrous P/M Compacts," International Journal of Powder Metallurgy, 27 [4] 339-345 (1991).

7. Madan, D.S. and German, R.M., "Structure-Property Relationship in Iron Compacts Alloyed with Boron," Advances in Powder Metallurgy, Vol. 1, Compiled by T.G. Gasbarre and W.F. Jandeska, Metal Powder Industries Federation, Princeton, NJ pp. 225-236 (1991).

8. Bose, A., Jerman, G., and German, R.M., Powder Metallurgy International, 21 [3] 9-13 (1989).