Substrate Selection for Tools Used with Hard Thin Film Coatings

Any discussion of tool steels for use with thin hard coatings must take into consideration coating conditions, especially high temperature processing, and post hardening conditions. Properties of tool steels can change drastically during coating and heat treatment. Therefore, substrate properties required when using coatings will be different than those employed for noncoated applications--both for tool development and use.

Thin Hard Coatings for Tooling

Methods used to produce thin hard coatings on tool substrates include:

Thin hard coating methods can be divided into two groups, based upon coating process temperature: a) high temperature process and b) low temperature process. This article will not discuss the PACVD method because it has not been used by American industry.

As currently used in the United States, CVD and TD are both high temperature processes. PVD is a low temperature process. In Japan and Europe, low temperature CVD and TD, as well as high temperature PVD processes, are available.

Fig. 1 - -Relation between coating temperature for thin hard coating and that for ordinary hardening.Coating & Substrate Hardening during Cooling (Q+T)

As shown in Fig. 1, high temperature coatings are done at a temperature compatible with the hardening (austenitizing) temperature of steels, and therefore need substrate hardening to occur during the cool down from coating temperature or with reheating. Process sequences for toolmaking are summarized in Fig. 2.

Fig. 2 -- Process sequences for tool making by thin hard coating.

CVD coating, especially for large tools, usually is done by sequences (C) and (E) since high enough cooling rate for quench hardening of most steels cannot be accomplished during cooling from the coating process temperature. Sequences (D) and (E) usually are applied to loosely toleranced tooling, since the size movement control can be more difficult due to the crystal structure transformation of substrate steels.

Hardening after coating procedures is not required for PVD coatings. However, the PVD temperature (not starting temperature, but true temperature during coating) should be selected relative to the optimum tempering temperature of each substrate steel used. Otherwise, substrate hardness after the coating operation could be lower than its original hardness.

CVD coating can be done using a wide variety of coating temperatures, and on a wide variety of substrates. However, switching of coating temperatures and gas composition to harmonize with the substrate materials being used is not easy to accomplish in production by commercial treaters. Therefore, commercial CVD coatings usually remain within D-series die steels, high-speed steels and cemented carbides.

Tooling Materials

Table 1 lists the types of tooling materials used for metalforming, their capability for thin hard coating, hardness in use and applications. Most tooling materials can be coated by at least one and usually all of the thin hard coating processes.

Key in selecting substrates and types of coating used is the substrate hardness after coating. It must be high enough to ensure support of the coating when a mechanical load is applied to the tooling during forming. In rare cases, coating of materials with low substrate hardness, such as copper alloys and cast iron, without hardening is successful when forming pressures are very low.

Material Properties Related to Tooling Failures

The most critical point of attention for all thin hard coatings is the role of plastic deformation of substrates. The potential for large plastic deformation of the substrate by high loads applied in metalforming can induce cracking in coatings that have low ductility. This damage results in peeling of the coating leading to wear and galling problems.

These types of failures are thought to be a failure of the surface coating, but in actuality are failures of the substrate surface directly under the thin hard coating. This misunderstanding usually results in discontinuing the use of coatings rather than changing the substrate to one that will support the coating.

Another failure in thin hard coated tooling results in spalling (peeling) or chipping of the coated layer. This causes damage to the product material that may not have happened with softer uncoated tool surfaces. A spalling problem frequently is encountered with PVD. Chipping will occur near the end of practical tool life with CVD and TD.

The plastic strain of a substrate in which cracking of the coating was confirmed can be concluded to be about one percent and is the same for both TD-coated steels and PVD-coated steels. Similar values can be expected for CVD-coated steels. It has been confirmed that there is not a large difference of the critical strain between TD-VC coatings formed on various steels, ranging from carbon tool steels to high-speed steels.

A shorter life of the thin hard-coated tooling compared with uncoated tooling is mainly attributable to spalling from poor adhesion strength of the coating, especially in PVD coatings. This is the result of either improper processing conditions or a failure of the coatings caused by plastic deformation of the substrates. These two important facts always should be kept in mind to obtain successful application of thin hard coatings.

Fig. 3 -- Correlation in occurrence of failures in coating, substrate and interface of thin hard coated materials.

Another point that should be made is the strong correlation in failures and damage phenomena between coating substrate and interface, as exemplified in Fig. 3. It is important to keep in mind that failures of tooling are induced by very complicated phenomena, and therefore the effect of substrate material selection on successful applications is very complicated.

Relation of Substrate and Properties of Coated Tooling

Roughly speaking, the surface properties of thin hard coated materials usually are independent of the type of substrates used so long as the substrates have enough strength. The change of substrate materials results in a change of the micro-structures of coated layers, such as size of crystal grains, preferred orientation of crystal growth, surface roughness, etc. Strictly speaking, the surface properties can change with the same kind of coating--even by the same coating method--when the substrate is changed. However, this problem will be ignored here since the problem may be very complicated because of the correlation between coating conditions and substrate materials.

Major properties, such as toughness and fatigue strength, are determined by those properties found in the substrate materials, which are inherent to each uncoated material. In some cases, the effect of coating procedures and the existence of a strong coating on substrates should be taken into consideration.

Interface properties can be determined by the cleanliness of substrates, especially with PVD, and the reaction between the coating and substrate during the coat formation, as well as the kind of substrate and coating material. From this point on, only those problems that are highly related to substrate selection will be discussed.

Fig. 4 -- Relation between compression strength and hardness of tool steels and cemented carbide.

The first consideration for good selection of substrate materials is that substrates have the hardness (compression yield strength) sufficient to prevent the substrates from being subjected to large plastic deformation (more than one percent). This hardness value can be roughly estimated from Fig. 4, which explains the relationship between steel hardness and compression strength; and Fig. 5, which introduces the hardness range usually selected for various tooling materials. One percent proof strength, not shown in Fig. 4, can be estimated roughly to be larger than 20 to 30 percent of 0.2 percent proof strength.

Fig. 5 -- Approximate hardness in use of various tooling materials.

A problem in substrate selection for metalforming applications is the difficulty of estimating the compression stress that actually is applied to each tool component. Note that the hardness of substrates after coating may be somewhat different from the original heat treating hardness.

Normally, it is said that PVD coating does not affect substrate hardness. However, hardness drops can occur. A larger hardness drop is observed on low alloy steels and no drop occurs with the high-speed steels listed in Table 1.

Other published results show about HV 150 drop on M2 high-speed steel; however, high-alloyed steels like high-speed steels are better selections to minimize a possible hardness decrease.

Hardness decreases at the substrate surface can be large with TD and CVD processing because of the decrease of carbon (decarburization) for formation of the carbide coatings. This phenomenon can be more serious in substrate materials containing only a small amount of "free carbon" at coating temperatures. The free carbon, carbon that easily can combine with atoms of carbide-forming elements (V, Ti, etc.), means the carbon in the austenite matrix of steels and carbon in the cobalt binder of cemented carbides.

The degree of hardness drop changes with substrate materials and conditions of coating and post hardening. No drop is observed in high-carbon, low-alloyed steel such as W1 and O1, and a large drop is seen with low carbon, medium alloyed steel such as S7.

In cemented carbide decarburization produces the special crystal phase called "h phase" and makes tooling very brittle. Selection of proper thickness and carbide without large deterioration is important.

Decarburization of steels does not make any special phase, for example an "h phase." However, the lower substrate hardness, as well as large tensile residual stress at the substrate surface decreases fatigue strength. The harmful effect of decarburization can be recovered, although not completely, during post hardening by thermal diffusion of carbon from the core of the substrate toward the surface.

Fig. 6 -- Fatigue strength of thin hard coated D2 steels

An improved strength derived from the existence of hard coatings on soft substrates is obvious in the fatigue strength results at cycles as shown in Fig. 6. CVD and PVD effectively increased cycles to failure. TD also improved strength of D2 but only when TD-coated D2 was tempered at high temperature. In the case of TD-coated steels, post hardening is a very effective method to improve fatigue strength. Therefore, post hardening is highly recommended for tooling used in severe working conditions where high compression stress is loaded onto tooling, resulting in fatigue failure of the tool.

Considerations for Selection

Proper substrate selection is accomplished by considering three major points. Remember, wear is the responsibility of the thin hard coating, not the substrate.

As shown in Fig. 7, deformation (shape change) is highly affected by cooling rate in the quenching operation. The slowest cooling rate can provide the smallest deformation. Consequently, air hardening steels most often are recommended for CVD and TD.

Deformation can be caused not only by improper quenching, but also by the heterogeneity of the substrate material structure. Powder metallurgy steels are highly recommended for tooling that requires minimum out of roundness, because of their very uniform distribution of fine carbide particles. Cemented carbides also are recommended for the same reason.

Furthermore, cemented carbides are the best material to minimize a size-movement problem. They eliminate the need to match good coating condition with preliminary hardening condition. This duplication of heat treating conditions is needed with steel substrates to minimize the change in the amount of retained austenite and martensite to control distortion before and after the coating process.

Normally, surface properties of coated materials are almost independent of the substrate material being selected. PVD does have a relatively large dependency of bonding strength between coatings and substrate materials. With PVD, the highly alloyed steels are said to create the best bond strength. For TD and CVD, dependency of bonding strength on substrate materials is so low that it can be eliminated as a selection factor.

Fig. 8 -- Comparison of wear resistance and toughness of some kind of tool steels (data from Crucible Steel's catalog).

Since wear is the responsibility of the selected coating, the information offered by material makers, as shown in Fig. 8 and Fig. 9, should be used to select materials with the highest quality in other properties rather than wear resistance. For example, if the tooling will be used under high impact conditions, A2 or CPM M4 are good candidates with substrate hardness of HRC 60 and 64, respectively.

If the tooling fails due to fatigue failure, the steels can be hardened to higher hardness by high-temperature tempering. Tempering at high temperature decreases residual tensile stress at the substrate surface to increase fatigue strength. Toughness is improved a great deal.

Also, high-temperature tempering can decrease the retained austenite in steel substrates. This results in higher compression yield strength and makes size movement control much easier.

Only some cold working die steels, such as D2 and D7, can be tempered at low and high temperature to reach hardness levels of HRC 57-62. Modified steels recently developed in Japan and the United States are highly recommended for TD and CVD coating. Higher hardness can be obtained easily by high-temperature tempering. Furthermore, they feature higher toughness, better machinability, deeper hardenability, etc., when compared to standard D2.

The selection of good quality materials is more important than the type of material selected for large tooling. Very large differences, much larger than those obtained between different materials, can be observed within a single type of steel. These differences mainly are caused by differences in the size and shape of carbide particles and their heterogeneous distribution (segregation) in steels. Therefore, powder particle steels usually are much better than wrought steels for large tooling.

Rating of Tooling Materials

Table 2 shows a rating of various tool materials after TD coating in terms of fatigue strength, toughness and compression strength. This can be a useful starting point for tool steel selection, when used in conjunction with other factors such as price, availability, machinability, ease of heat treatment, etc. Please keep in mind the ranking is a simple guideline to be used as a rough comparison of materials, and that the properties of materials are highly influenced by heat treating condition and the quality of bar stocks, etc.

This can be mitigated by post hardening. Item D may be effective only by high-temperature tempering and post hardening.

The ratings for TD-coated materials are not far from that of CVD-coated materials. In CVD, unlike TD, carbon atoms for carbide formation are supplied from both the substrates and the coating medium (gas). However, fairly large amounts of carbon atoms in substrates are consumed to develop a carbide layer. This can cause a decarburized layer and residual tensile stress at the substrate surface. Therefore, the ratings for TD-coated materials shown in Table 2 can be used for CVD coating with little or no modification.

The rating for ordinarily hardened materials, shown in Table 3, can be applied to PVD with the addition of an evaluation on bonding strength of each material, which also is highly dependent on proper PVD coating conditions, as long as the PVD coating could be done with no substrate hardness drop.

It is quite difficult to make a similar ranking of cemented carbides and other tooling materials, due to a lack of information on the properties of each cemented carbide after thin hard coating. The ranking of these materials may not be as important as with steels. These are worth using only in the limited areas, for example, when the tooling is to be used under very high loading or the tooling requires extremely tight dimensional tolerance.

However, it should be determined which commercial brands of cemented carbide are more susceptible to brittle "h phase" formation in CVD and TD coating.

Conclusions

Thin hard coating processes (CVD, PVD and TD) can produce coatings of pure carbides and nitrides onto a substrate surface. Because of the excellent resistance of these coatings to wear and galling, they effectively eliminate surface damage problems to product produced by metal form tooling.

With the elimination of wear and galling problems by thin hard coatings, substrate selection can be focused on solving tooling failures caused by insufficient toughness and fatigue strength.

Selection of proper substrate materials for fatigue and toughness combined with proper selection of coating and post hardening conditions should offer the optimum in tooling performance.

Since the surface properties of coated materials are independent of the substrate materials used, a large possibility exists for cost reductions through the selection of less-expensive substrate materials that have less wear and galling resistance. These less-expensive substrate materials should have better machinability and heat treatability, along with good toughness and fatigue properties.