This work investigates the reliability and reproducibility of scratch testing for YSZ and TiO 2 coatings deposited on NiCr bondcoats by plasma spraying. Scratch tests were conducted on cross-sections of the spray coatings. An image of the scratched area was taken after testing using an optical microscope in order to determine failure mode. With a statistical evaluation, the adhesion and cohesion strength were determined. The results show that cross-sectional scratch testing is effective for estimating the adhesion and cohesion strength of plasma spray coatings.

Titanium oxide coatings were suspension plasma sprayed onto different substrates. The suspension was formulated using fine rutile pigment in the mixture of water with ethanol. The zeta potential of the suspension was determined. The spray process parameters were designed using a full factorial plan using spray distance and torch scan velocity as the variables. The temperature at spray process was monitored using a pyrometer. The X-ray diffraction (XRD) analysis enabled to find out the crystalline phases in sprayed deposits and, in particular, the anatase content. Scanning electron microscope (SEM) enabled to characterize the coatings’ microstructure. The coatings included well molten lamellas and zones of loosely agglomerated and sintered grains. The scratch test of the coatings enabled to determine their mechanical properties such as critical load and scratch hardness.

Plasma sprayed ceramic coatings have been widely applied in modifying surface properties of metal components. They are useful to prevent various types of wear, corrosion, erosion and thermal degradation, thereby extending components service life and reducing the need for expensive and repetitive maintenance. The durability and functionality of plasma sprayed ceramic coatings is critically dependent on the adhesion between the ceramic coating and the underlying substrate as well as the cohesion between splats. In this work, both nanostructured and conventional Al 2 O 3 coatings were prepared by atmospheric plasma spraying technology (APS). For each feedstock, four kinds of coating samples deposited under different spraying parameters were designed, and moreover, seven groups of conventional coatings with different thickness were deposited under the same spraying parameters. Adhesion/cohesion of the plasma sprayed Al 2 O 3 coating samples were evaluated by scratch testing. The results obtained reveal that the spraying parameters have strong influences on the microstructure of plasma sprayed Al 2 O 3 coatings, which in turn influence their properties including deposition efficiency, porosity, microhardness as well as adhesion/cohesion.

Controlled scratch testing, dry abrasion tests and hardness measurements were performed on WC-12Co coatings produced by the high velocity oxy-fuel spraying of nanostructured and conventional feedstock powders. The information obtained employing these different evaluation techniques was used to provide insight into coating behaviour and identify the most abrasion-resistant coatings. The results indicated a correlation between scratch hardness and the microhardness determined by Vickers indentation for the coatings. There was good agreement between the scratch test and the abrasion test in identifying the best coatings for use in dry abrasion. Observation of the scratched surfaces and wear scars indicated material removal by splat debonding and fracture. The results demonstrate the usefulness of the scratch test for assessing the abrasion resistance and wear behaviour of coatings.

This work examines nanocrystalline thermal spray coatings prepared under several power and stand off distances. The coating material was nanocrystalline alumina + titania and zirconia (ATZ) powder mixture deposited using an air plasma spray process. The experimental design employs a full factorial method totaling four samples. The mechanical properties of four coatings were compared via scratch and hardness testing. The scratching process was monitored for transverse loads during the test and the variation in load monitored. The profiles of scratches were analyzed using white light interference profilometry and studied further via SEM. The results indicate that the microstructure reflected features observed in the scratch test. It was also shown that the scratch process and morphology depend on the normal load and spray process.

This paper compares the wear properties of HVOF-sprayed WC/12Co hardcoatings produced from different powder feedstock materials, including conventional, nanophase, and mixed powders. The mixed or multimodal feed powder is designed to minimize the amount of material that goes through a high temperature cycle during spraying, thus potentially limiting the extent of decarburization in the resulting coating. As will be shown, decarburization is indeed minimal in a multimodal coating, which translates into exceptional resistance to abrasive and sliding wear. Another favorable factor is the ability to increase the volume fraction of hard WC phase in such a coating, thereby further enhancing its wear resistance.