Modeling of Thermal Stresses in Composite Diamond Coatings and Mechanisms of Improvement of Adhesion
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coated tools, it is found that the adhesion of CVD diamond films on tools is an important property, and the adhesion can be improved by using an interposing layer [3]. In the present work, a composite diamond film is deposited on tool substrates. In order to understand the mechanisms of adhesion improvement and to reach the optimum design of composite coatings on tools, a two-dimensional finite element analysis of thermal stresses in the composite diamond films and interfaces was performed.
[ J Diamond
--
S1-
8- ..
20 Y • X
...... F x TiC or TiN '-'-'
WC(Co) or SiN
Fig. 1 Two-dimensional model of composite coatings on tools (dimensions in litm) 847
Mat. Res. Soc. Symp. Proc. Vol. 356 0 1995 Materials Research Society
Table 1.List of material parameters Material
Thermal expansion
WC IS iN -Diamond
coefficient (10- 6/oC)
4.6 12.4 2.1 5.4
Elastic modulus (1011 Pascal)
ITiC ITiN
3.1 9.7
7.4 4.6
9. 2.9
MODELING PROCEDURE The dimensions of the specimen used in the present analysis are shown in Figure 1. WC(Co) and Si 3N 4 tool substrates were used. A discontinuous interposing layer, TiC or TiN, was considered. All external boundaries were free so that bending was allowed during cooling from 900 0C to 0°C. In this modeling, uniform cooling with perfect bonding at all interfaces and without any creep deformation in the films was assumed. Numerical solutions were obtained with COSMOS software [4]. Because the thermal expansion coefficient is dependent not only on the material but also on the temperature, an average value in the temperature range from 9000 C to 00 C was used [5]. All the parameters used in the analysis are shown in Table 1. All the materials are assumed to have the same Poison's ratio, 0.3. The ratio of the thickness of the substrate to that of the film is assumed to be 5. When this ratio is larger, the distribution of stresses in the film and the substrate is not found to change very much. Therefore, the calculated stresses are representative of thermal stresses in coatings on tool substrates. In the experimental wear test that we have performed to determine the adhesion and wear resistance, the diamond films were polished against diamond paste wherein tangential forces were acting on the diamond film. These tangential forces were represented by Fx (100 MPa) applied on the top surface in the model, as shown in Fig. 1. In order to equalize the Fx, -Fx (100 MPa) was applied on the bottom surface, as shown in Fig. 1. Under these two forces, the substrate tends to rotate. Since the rotation does not give rise to any additional stresses, this model represents the stresses generated under simple shear by a wear test. Also for comparison, analysis of stress in a single diamond layer of thickness 4 pm on tool substrates was also carried out. RESULTS AND DISCUSSION Figures 2(a), (b), and (c) show the distribution of shear stress TauXY in the WC(Co) tools coated with the single diamond layer, the TiC composite diamond layers, and the TiN composite diamond layers, respectively. For the single laye
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