Adhesion of electrolessly deposited Ni(P) layers on alumina ceramic. I. Mechanical properties

The adhesion mechanism of electrolessly deposited Ni(P) on alumina ceramic substrates has been investigated. The adhesion was measured by direct pull-off tests and by 90° peel tests, which provided information on adhesion strength and fracture energy, respectively. An assessment is made of quantitative aspects of both adhesion measurement techniques. The observed mechanical behavior is rationalized using the Griffith–Irwin theory. Two types of alumina substrates with different roughnesses were used. Ni(P) was deposited from two types of electroless Ni(P) solutions, one with glycine and one with acetate as the complexing agent. The fracture surfaces were analysed with scanning electron microscopy, combined with energy dispersive analysis of x rays. The adhesion strength of the glycine-type Ni(P) was much higher and the fracture energy was lower than that of the acetate-type Ni(P), for both substrate types. This implies that the difference in adhesion strength is not caused by differences in interfacial chemical bonding, but rather by differences in flaw sizes. Since high adhesion strength was measured on smooth substrates, along with low peel strength, it is concluded that strong adhesion can be obtained without making use of mechanical interlocking. Further research should be aimed at controlling the interfacial flaw sizes.