Tensile Strength of coating Adhesion

General Description

This Test method is a modified standard test method used for the tensile testing of Ceramic coatings on a metal substrate.

The standards that are used are the ASTM Standard test method for the Tension Testing of Porous Metal Coatings on Metal Substrates. ASTM F1147-88. There is no equivalent British Standard test method.

See Copy enclosed Appendix 1, to understand variations to Test Method described hereafter.

Variations. Introduced for the testing of Hydroxylapatite Ceramics [HA]

See Paragraph 7.1.2. ASTM F1147-88 and Note 1

Due to the problems of glue penetration yielding false strength results (especially when thin coatings require analysis) the following rationale has been adopted in order to produce results which are independent of glue interference.

A series of batches of pull off experiments shall be conducted, using a different thickness of coating in each batch. The mean strength of each coating of given thickness shall be monitored and plotted on a graph with the following axes:- Y-Axis = Mean strength of given batch. X-Axis = Mean Thickness of coating on given batch.

Graph using idealised data (not shown here)

Interpretation of the graph.

It is obvious that with a coating thickness of zero microns that we will be measuring the strength of the glue. However as the coating thickness increases, we will not be sure whether or not the glue will have penetrated the coating or not. We will probably be measuring the strength of a mixture of glue and coating, until the coating becomes sufficiently thick for the glue not to penetrate to the substrate [we shall define this as the 'Glue' line]. This will depend on the viscosity of the glue, and the porosity of the coating.

As can be seen from the graph there reaches a point where the slope of the graph changes abruptly, and the strength of the coating continues to decrease but at a slower rate. [We shall define this as the Coating Line]. We are at this point onwards (increasing thickness) measuring the strength of the coating alone.

If we wish to give the strength of a coating which is only, say 50 microns thick, we cannot read it directly from this graph because that is in the region of the graph where there is interference from the glue. We must extrapolate the coating line backwards towards the Y-axis and estimate the strength from that line.

A given type of substrate material should be specified for these tests.

Results should be all be given, however if for some reason some are deemed to be void; they need not be used in production of the graph providing the explanation of the reasons is given. For example, the glue is defective, or there is a reduced area of adhesion. etc.

NOTE: Glue penetration of the coating to the substrate is NOT grounds for discounting data using this method.