Coating Design

How such a coating configuration (see Fig. 1) would satisfy the five critical requirements listed above is briefly discussed below.

Adherence — Strong adherence of the proposed coating is based on formation of a chemical bond at the interface as a result of the growth of a new phase. Such formation of interlayers to enhance adherence of chemical vapor deposition (CVD) coatings is well established. However, diffusion of species between the substrate and the coating during the deposition process can lead to several deleterious effects, such as Kirkendall porosity or the formation of brittle phases (5). Hence, controlled diffusion of selected species is essential in forming an appropriate interface between substrate and coating. This can be accomplished in the CVD process by varying the coating parameters. An example of enhanced tungsten diffusion into a titanium carbide (TiC) coating leading to the formation of (Ti,W)C during the CVD of TiC onto a WC-Co cemented carbide is shown in Fig. 2. Under typical coating conditions, such a distinct tungsten-rich zone is not obtained.

The substitution of Al for Si and 0 for N in a-or p-Si3N4 results in the formation of solid solutions termed a or p SiAIONs, respectively (6, 7,8). By selection of an appropriate deposition temperature, it is expected that a SiAION-type compound can be formed at the coating/substrate interface either by chemical interactions between the AIN/AlxOyNz layer and the substrate or by reactions between the gases used in the CVD process and the substrate. Thermodynamic modeling of the substrate/CVD reactive gas mixture systems has indicated that new phases can be formed at the interface under some conditions (9). For example, when monolithic SN (Si3N4 + V2O3 + AI2O3) is exposed to the gas mixture used to deposit AIN, a silicon oxynitride phase is predicted to form at typical CVD temperatures. This phase is the basis for some SiAION-type compounds. The formation of such a compound is expected to improve adherence as a result of chemical interactions between the coating and substrate.

Additionally, such an interface would also minimize residual stresses at the interface.

Since the coating is compositionally graded and has no distinct interface between the AIN/AlxOyNz layer and the AI2O3 + Z1O2 layer, no adherence problems between the two layers would be expected.

Residual Stress — Thermal expansion mismatch between the coating and substrate (see Table 1) can lead to high tensile or compressive stresses in the coating if the linear thermal expansion coefficient of the coating is higher or lower, respectively, than that of the substrate. If these stresses are large, crack formation through the thickness of the coating and/or the coating/ substrate interface can occur. Furthermore, large differences between the thermal expansion coefficient of the substrate and the applied coating can result in de-bonding and, therefore, poor adhesion. For identical substrates, thicker coatings typically contain a higher density of cracks, resulting in a decrease in the bend strength and

2 pm

2 pm

Diffusion zone at the Interface between a TIC coating and a WC-Co cemented carbide substrate.

Figure 2.

Diffusion zone at the Interface between a TIC coating and a WC-Co cemented carbide substrate.

fracture toughness of the coating/substrate system with increasing coating thickness. Residual stress and large differences in linear expansion coefficient between coating and substrate can result in thermal fatigue under the thermal cycling conditions encountered in heat engine applications.

These problems can be minimized by forming a solid solution or new phase at the coating/substrate interface and by continuously varying the composition of the coating to give it a low internal residual stress. A continuously graded coating also reduces the effects of thermal shock due to thermal expansion mismatch between the various materials.

The concept of interface tailoring has been successfully demonstrated for the application of wear resistant coatings on monolithic silicon nitride and silicon nitride composite substrates (10). These coatings have been successfully tested in metal cutting applications, where the coating/substrate system is exposed to similar conditions as are encountered in heat engines: high contact stress, high temperatures, and thermal cycling.

As part of the design of the coating configuration, finite element analysis was used to predict residual thermal stresses in the coating and substrate. Results indicated very high residual stresses on both SC and SN substrates, suggesting that cracking might be a problem. Some calculated normal stress values for various AIN coating thicknesses on SC and SN substrates are given in Table 2. Three sets of stresses were calculated for SC to accomodate the range of thermal expansion coefficient values reported in the literature. The calculated stress values for a SN substrate exceed the room temperature modulus of rupture of AIN (400-450 MPa) by nearly a factor of two. These

Table 1

Physical properties of candidate materials (Taken from Engineering Property Data on Selected Ceramics, Vol. l and III, MCIC Report, March 1976 and MCIC Report, July 1981)

Table 1

Physical properties of candidate materials (Taken from Engineering Property Data on Selected Ceramics, Vol. l and III, MCIC Report, March 1976 and MCIC Report, July 1981)

Property/Material

0 0

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