Technology Briefing 554
F J Blunt
Grit blasting substrates prior to spraying is routine, but which grit is best? The ASTM adhesion test is well respected, but is there an alternative?
Background
Sprayed coatings are variously used to delay corrosion or wear of components in service and one of the most fundamental requirements is good adhesion to the substrate. Alumina has several excellent properties which can be exploited when it is applied as a surface coating. It is resistant to many corrosive liquids, is a good electrical insulator, and has good resistance to abrasive wear.
For a coating such as alumina, it is generally believed that the adhesion of the coating is strongly related to mechanical keying between the sprayed particles and the substrate surface, and hence grit- blasting of the substrate surface is routine preparation for spraying. To date, no systematic study has been carried out to find out whether there is an optimum roughness, or whether other parameters, such as fuel gas or interpass temperature and time have a significant effect on the adhesion of high velocity oxyfuel (HVOF) sprayed alumina coatings.
The most common test of adhesion is the ASTM C633-79 tensile test. This test is adequate for many types of coating, including ceramics and most metals. However, it is relatively expensive, slow and it requires a special test geometry. An alternative, more rapid test is available commercially (PAT tester), and this was chosen for comparison. This test is in principle very similar, but the geometry of the test allows it to be carried out directly on sprayed components provided there is sufficient flat area on which to place the jig.
Objectives
- To identify the best method of preparing the steel surfaces prior to spraying of alumina.
- To determine whether the particle velocity, fuel gas or interpass time had a significant effect on adhesion.
- To determine whether the PAT test method for coating adhesion gave comparable results to the ASTM C633 pull-off test.
Experimental approach
Steel substrates were blasted using seven different grits. These included chilled iron, silicon carbide and alumina, each in more than one sieve size. This gave a range of surface roughness and deformation, which was characterised by profilometry and hardness measurements.
For the majority of the experiments, propylene was used as the fuel gas, using spraying conditions which are recommended by the manufacture of the equipment. Several variants were investigated:
- A longer barrel, which increases the particle velocity.
- Acetylene fuel, which reduces the particle velocity, increases the flame temperature, but reduces the power input to the particles.
- Very short interpass times, allowing the samples to become 'blue'. For each experimental condition, ASTM and the alternative PAT adhesion tests were carried out.
Results and discussion
The finest gritted surfaces gave the best adhesion (as much as 50% better bond strength than the roughest surfaces). It had been anticipated that increasing the velocity of the sprayed particles by using the longer barrel would also bring about an improvement in adhesion, but this proved not to be the case. It is known that particle velocity has a significant effect on adhesion, and this result suggests that the benefits of high velocity are fully attained with the standard gun design using propylene fuel.
Acetylene fuel gas gave coatings of poorer adhesion, but it was not possible to decide whether this was due to the reduced particle velocity or the lower power input to the particles.
An unexpected result was obtained with the samples using very short interpass times. These specimens had 30-50% better adhesion than the standard samples of comparable roughness. It is not standard practice to allow samples to get so hot during spraying that they become blue with oxide, nor is it standard practice to rigorously control interpass time. The cause of this improvement is not certain.
The PAT tester performed consistently, giving similar values for adhesion to the ASTM test, and ranking the coatings in the same order.
Main conclusions
- Surface roughness of the substrate is an important parameter in determining coating adhesion
- The finest gritted surfaces give the best adhesion of alumina coatings
- Propylene gives alumina coatings of superior adhesion to acetylene
- Increasing the particle velocity by adding a long barrel produces no further improvement in adhesion
- Short interpass times produce coatings of superior adhesion
- The PAT tester performs consistently, and ranks coatings in the same way as the ASTM test.
Recommendations
The finest grits, such as silicon carbide BSC60, BSC36 or chilled iron G12, should be used in preparing surfaces for spraying with alumina. This work also suggests that the preparation of substrates for other coatings also deserves a systematic study.
The PAT tester is recommended for rapid adhesion testing, as an alternative to the ASTM test.
