J O U R N A L O F M A T E R I A L S S C I E N C E L E T T E R S 8 ( 1 9 8 9 ) 3 4 6 - 3 4 8

Measurement of cohesion and adhesion strengths in alumina coatings produced by plasma spraying

E. LOPEZ, F. BELTZUNG, G. ZAMBELLI Department of Materials Engineering, Swiss Federal Institute of Technology, Lausanne, Switzerland

Oxide powder deposition by plasma spraying gives rise to a heterogeneous and porous layer. This layer consists of plate-like grains and a reliable measure- ment of their cohesion is an important parameter in characterizing the quality of a coating. Further, cohesion in the layer determines the wear behaviour.

Several methods have been proposed for measuring adhesion between the substrate and thin or thick coat- ings [1]. The most interesting ones are those which are based on the progressive scratching of a thin layer together with the respective recording of an acoustic signal which corresponds to gradual detachment of the layer [2].

An original method is proposed which enables measurement of the cohesion when scratching is per- formed from the substrate to the free surface of the layer. The test apparatus used is a sclerometer which allows fixed-depth scratching and concurrent tangen-

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Figure 1 Evaluation of the cohesion strength in a plasma-sprayed AI203 layer. (a) Scratching from the steel substrate into the layer until it leads to half-cone shaped cracking. (b) Normal and tangen- tial forces (F, and Ft) plotted again scratch length. Indentation depth h = 10.6/~m. Vickers pyramid indentor.

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Figure 2 Schematic view of the groove and the half-cone with some geometrical parameters.

tial and normal force recording, while the affected surface is in translation [3].

In the present letter, the studied samples are restricted to a fine-grained dense hot-pressed alumina and a plasma-sprayed alumina coating.

The hardness, HV~ is calculated from the unloading curve recorded during classical Vickers indentation tests [4]. With increasing penetration depth (increasing load), HVi proportionnally decreases, as is often observed for materials which undergo elastic recovery. Results of the hardness tests, for a penetration depth of h = 10#m are reported in Table I.

The scratch hardness HV~ = Fn/A, is, by defi- nition, the ratio of normal force, F~,, to the projection of the Vickers pyramid contact area on the surface of the sample, An. Scratching is performed with the edge of the Vickers pyramid. The rake angle is 22 ° and the displacement velocity is 4 #m sec- '. The scratch hard- ness varies depending on the penetration depth, h, in a similar way to that of the static hardness. These

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Figure 3 Schematic view of a pyramid diamond showing the reaction forces while scratching is performed.

0261-8028/89 $03.00 + .12 © 1989 Chapman and Hall Ltd.

T A B L E I Selected data for the characterization of wear behaviour

Process Phases Grain size HVi(10 #m) HVr(10/~m) T(10 #m) L ( 5 N ) (#m) (GPa) (GPa) (MPa) (~m)

Hot-pressed A1203 (ct) + 500p.p.m. MgO 1.1 20 10 100 70 Coated A1203 (7 + c0* - 7/5 t 5 40 117

*y = main phase. ?Vickers test performed on a perpendicular section of the layer.

results show that HV~ and HVr are significantly smaller for the coating compared to the sintered a lumina (Table I).

Fig. 1 shows a schematic description of the original method which permits the cohesion of the plasma- sprayed alumina layer on a steel substrate to be evaluated. Perpendicular to the layer, scratching is performed with the typical groove formation from the substrate to the surface of the layer. In the vicinity of the latter, at a critical distance L, scratching leads to half-cone shaped cracking (Fig. 1 a). This failure mode is brought to the fore on the diagram (Fig. lb) where the normal and tangential forces are recorded against the scratch length.

The geometrical parameters involved in the calcu- lation of the decohesion shear stress of the half-cone are given in Figs 2 and 3. The apex angle, 20, of the cone remains roughly constant, between 100 ° and 120 ° , and is independent of the penetration depth (h < 20 #m).

Recording of the normal and tangential forces allows calculation of the angle, c~, between Fn and the resultant force, R. It is found that ~ is approximately equal to the complementary angle of the Vickers pyramid's half apex angle W = 68 ° (Fig. 2). Because the half apex angle of the half-cone is smaller than W, R is not parallel to the half-cone generating line l (Fig. 3).

A critical shear stress, T, characterizing the material's cohesion strength can be calculated using the expression T = 2R'/S, where R' is the projection of R on the generating line and S = nrl, the surface of a circular cone. Plots of the critical shear stress T against the penetration depth, h, for both materials are shown on Fig. 4 in a log-log scale. It can be identified that T = kh m, where m < 0.

The values of T for h = 10/~m can be adopted as a conventional value for the cohesion of the brittle microstructure. These are given in Table I and it is found that T (10#m) equals 40 and 100MPa for plasma-sprayed and hot-pressed alumina, respectively. The correlation factor appears to be identical to that calculated for the scratch hardness (about 2).

Moreover, it must be pointed out that at the inter- face between the metallic substrate and the coating, a crack is to be seen for a critical penetration depth hc > 10ktm. This suggests that scratching across the interface can lead to a local decohesion of the layer. Making the assumption that the crack's pattern is a half-penny shape of radius c, the possibility is given to evaluate the adhesion toughness by calculating a criti- cal stress intensity factor, K~, in the same way as it is carried out after indentation.

Two simultaneously acting processes are damaging the surface of the ceramics: split up on a grain-size scale (microdamaging) and chip formation due to lateral and radial cracks on a groove-width scale (macrodamaging).

The half-cone shaped cracking seems to originate from radial cracks formed in front of the indentor as the surrounding stress field reaches the free surface. At that point, an inversion of the stress field occurs in the front end of the groove which causes the cracks to propagate, running along the maximum tensile stress plane. Consequently, the apex angle of the half-cone will basically be related to the indentor's geometry. This can explain the geometrical similarity of the half- cones for both types of microstructure. It turns out that the creation of the half-cone will predominantly be affected by the resistance of crack growth. Attempts to obtain a better understanding of these mechanisms are being made.

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Figure 4 Variation in log-log scale of the critical shear stress, T, to develop a half-cone with indentation depth. (zx) Fine-grained hot-pressed alumina, (o) plasma-sprayed alumina coating.

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The use of the shear stress in measuring the cohesion is based on an elementary model and on assumptions which have to be checked. In this con- text, further parameters which enable the inves- tigation of cohesion can be established. This is the case, for instance, of the height L of the half-cone in connection with a conventional normal force, say Fn = 5 N. Fig. 5 shows the cone height or the critical length, L, plotted against the normal force, Fn. This plot allows the determination of L(5 N) (Table I).

In conclusion, the method presented enables oxide cohesion measurements by calculation of the shear stress, T, which initiates a half-cone shaped crack. It has been verified that the cohesion is stronger in the hot-pressed alumina relative to the plasma-deposited layer. The next steps will consist of a theoretical analy- sis of the mechanisms responsible for the decohesion of the half-cone. Put into practice on thick and brittle coatings, this interpretation of scratching offers an

Figure 5 Variation in log-iog scale of the critical cone height, L, with normal force. (~x) Fine-grained hot- pressed alumina, (o) plasma-sprayed alumina coating.

interesting method of quality and reliability control, in particular in the development of new coatings,

A c k n o w l e d g e m e n t

The authors thank Plasma-Technik AG, Wohlen, Switzerland, for providing the plasma-sprayed alumina-coated samples.

R e f e r e n c e s 1. P. LAENG, P. A. S T E I N M A N N and H. E. HINTER-

MAN, Oberfliiche-Surfaee 4 (1982) 108. 2. J. VALLI, U. M A K E L A , A. MATTHEWS and V.

M U R A W A , J. Vac. Sei. Teehnol. A 3 (1985) 241. 3. E. LOPEZ and G. ZAMBELLI (1988), to be published. 4. J. L. LOUBET, ThOse 3 + cycle, Universit6 de Lyon I, no.

1332 (1983).

Received 27 July and accepted 6 September 1988

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