For many years, thermal spray coatings have been employed for improved thermal, wear and corrosion protection in many industrial domains. Thermal spray coatings are routinely used in many applications including aircraft engines, power plant turbines and on pulp rolls in the paper industry, where high temperature damage or extensive wear occur. Plasma spraying (water or gas stabilized), wire arc, high velocity oxy-fuel (HVOF), detonation gun and flame spray are the most common deposition methods of thermal spray coatings.
In order to ensure optimum function of coatings, the associations between the mechanisms of coating formation and its mechanical properties should be known. Although the mechanisms for formation of these coatings have been extensively studied, obtaining comprehensive information on mechanical properties continues to be difficult due to the heterogeneity nature of the coating (Figure 1).
Figure 1. Typical microstructure of WC-17Co HVOF-sprayed coating showing WC grains and binding metal matrix (Scanning Electron Microscope image).
So far, macro scale methods such as four point bending or microhardness at relatively high loads have been used for measurements of mechanical properties. Such methods measure 'composite' properties of the coating, but they disregards the strongly heterogeneous structure containing a softer binding matrix and hard particles. The situation is even more difficult for cohesion and adhesion testing because one of the few standardized tests is tensile testing, by brazing or gluing a pair of samples and then pulling them apart. This method not only makes the evaluation of coating adhesion relatively difficult but it is also restricted by the tensile strength of the braze/glue.
This article presents a research project that deals with the latest methods for testing the mechanical properties of thermal spray coatings using instrumented indentation and scratch testing. The results of low load indentation and scratch tests on HVOF coatings are presented.
Thermal Spray Coatings: Heterogeneous Material
A key factor for determining the indentation parameters is the analysis of the grain size of thermal spray coatings. The properties of the individual splats or grains can be measured at very small loads of a few milinewtons, and when the load is increased a larger volume is involved which shows a ‘composite’ value of mechanical properties. This composite value ignores the material’s heterogeneity and until recently, this type of measurement has been routinely carried out. Yet, to better understand the relationship between the coating functionality and the deposition parameters, it is important to know the material’s properties on different scales. Measurement of these properties has been possible only lately using instrumented indentation and advanced automated matrix measurements.
In addition, due to nonstop recording of indentation depth and force, it is possible to calculate several other key characteristics of the material (besides hardness) such as elastic modulus and the plastic and elastic part of the indentation work. This provides a better insight about the elastic-plastic behavior of the coating, which can be closely associated with its wear and failure resistance.
Instrumented indentation on thermal spray coatings
Anton Paar indentation machines enable measurement at different ranges:
- for properties of single grains or splats at low loads, the Nanoindentation tester (0 - 500 mN) was used
- for the ‘composite’ properties at higher loads, the Micro-Indentation tester (0.03 - 30 N) was used
Instrumented indentation is not restricted by optical measurement of the imprint diagonal (although traditional Vickers hardness can be measured from the residual imprint depth) and it gives other useful information, for example elastic modulus and creep properties. The fundamentals of instrumented indentation are given elsewhere [1-3]; this article focuses on its specific application to thermal spray coatings.
Cohesion and Adhesion Tests by Scratch Testing
In recent years, attempts have been made to find out the adhesion of thermal spray coatings by applying scratch tests in the same way as in the thin film domain, that is scratching the coating’s top surface with increasing load. However, the method has been found to be unsatisfactory because of extreme surface roughness and thickness of thermal spray coatings. Here, a method is proposed which involves running a continuous load scratch test on a cross-section of the coating. Lopez et al.  initially proposed this method. The sample is cross-sectioned, integrated in resin, and then polished. The scratch is performed under constant load, and the indenter travels from the substrate via the coating into the resin, where the sample is embedded – refer Figure 2 and  for schematic explanation. The test is again preformed at a number of loads, and this is followed by calculating the projected area of the cone, Acn, extracted by the indenter. Two types of failure are usually seen - the cone originates at the interface between the substrate and coating and the cone originates in the coating. The adhesion of the coating can be characterized in the former case, while the cohesion of the coating can be characterized in the latter case.
The study uses samples that were sprayed from Cr3C2-25%NiCr, WC-17% Co¸ and exclusively developed (Ti,Mo)(C,N)-39%NiCo powders. The coatings will be further known as Cr3C2-NiCr, WC-Co, and (Ti,Mo)(C,N)-NiCo, respectively.
Figure 2. Schematics of the constant load scratch on a cross-sectioned
At the H.C. Stark GmBh (Goslar, Germany), the (Ti,Mo)(C,N)-39%NiCo powder was exclusively developed for use in sliding applications and initially tested by L-M Berger . were sprayed with previously optimized parameters at Skoda Research, Ltd. in Plzen, Czech Republic, Praxair JP- 5000 HVOF system was used to spray all the samples with previously optimized parameters. The coatings’ thickness ranged between 300 µm and 500 µm. All of the samples were sectioned, embedded in LECO resin (cat.no. 811-563-101) and then metallographically polished.
Shown in Figure 1, the microstructure of all coatings containing grains with dimensions of a few micrometers (WC-17Co), less than 3 three µm ((Ti,Mo)(C,N)-NiCo) and approximately 10 µm Cr3C2-NiCr) and dispersed in a metallic matrix.
Indentation Procedure and Results
An Anton Paar Nanoindentation Tester (NHT) was used to perform instrumented indentation at four maximum loads of 2 mN, 20 mN, 100 mN and 200 mN. All indentations were performed on the coating’s cross-section. The loading and unloading time took 30 seconds without any pause at maximum load.
Preselection of the precise indent positions were done using the “Visual Advanced Matrix” mode  which enables precise selection with the integrated optical microscope, as shown in Figure 3. The indentations were then automatically performed on these spots and the areas to indent on all tested coatings were chosen so that they were as homogeneous as possible, that is, without any visible pores and splat or grain boundaries. At each of the four selected loads, 10 measurements were made to raise the statistical reliability of the results. Outlaying values caused by presence of pores and heterogeneties invisible under the optic microscope were rejected.
Figure 3. Visual Advanced Matrix mode showing the precise position of each indentation test.
Figure 4a shows hardness results that clearly reveal the decrease in hardness with increase in the maximum indentation force. This trend can be better explained by the amount of the material affected by the indentation process. At small loads, the volume involved in the indentation process (related to maximum indentation depth hmax, see Figure 4b) match with the size of individual splats or grains composed of hard metals or carbides. However, in small volumes the measurements are not influenced by pores or grain boundaries and the properties well describe homogeneous material of single grains or splats.
As the indentation forces increase, the volume of material affected by indentation increases and also comprises of the softer metallic matrix. This causes the observed decrease in hardness as the indentation force increases. It is not possible to explain the anomalous behavior of the experimental (Ti,Mo)(C,N)-NiCo coating, although it is assumed that due to the extremely small size of the hard particles in this coating, the indentations at 2 mN were carried out in the particles as well in the softer matrix.
The increase of hardness at 20 mN load is a very likely result of pushing the hard particles into the softer matrix during the indentation which gives higher values of hardness. This remarkable phenomenon will be dealt with in more details in future experiments.
At the same time with hardness, the elastic modulus values were measured using supposed Poisson’s ratio of 0.3 and the results are illustrated in Figure 5. Interestingly, in contrast to hardness, elastic modulus continues to remain rather constant for all coatings, regardless of the highest indentation force.
At higher loads (1 N to 10 N), a slight decrease in the elastic modulus was noted. However, decrease in hardness at such high loads was observed to be less significant than at low loads.
Figure 4a. Decrease of hardness with increase of maximum indentation force for the three tested coatings.
Figure 4b. Maximum indentation depth (hmax) as a function of the maximum indentation force. Results from the Anton Paar Nanoindentation Tester (NHT).
Figure 5. Elastic modulus as a function of maximum indentation force.
Work of Indentation ηIT
The ratio of elastic to total work of indentation, ηIT, is a critical material property. This parameter (defined by the ISO 14577 standard) appropriately characterizes the elastic-plastic properties of the material. With regard to thermal spray coatings, the value of ηIT changes considerably with the amount of material involved in the indentation process. At small loads, the deformation field extends merely to the individual splats or grains as chosen by the Visual Advanced Matrix mode. These measurements demonstrate extremely high elasticity when compared to the coating as a whole. It is seen that the value of ηIT for all three tested coatings was relatively higher at lower loads and gradually decreased with increasing loads (see Table 1).
Table 1. Ratio of elastic to total work of indentation ηIT in percents.
When indentation is performed at low loads in the relatively uniform microstructure of single grains containing carbides, it leads to a high ratio of elastic to total work of indentation. However, at higher loads (above 100 mN), increasing contribution of plastic deformation in the metallic matrix and formation of intersplat and intergranular cracks results in dissipation of the indentation energy, which is reflected in lower fraction of elastic work of indentation.
Above 1 N loads the ratio of elastic to total energy of indentation almost does not change and remains about 30%. It is interesting to note that at loads above 1 N, the ηIT value was quite similar for all three tested coatings. At the lowest load, the Cr3C2-NiCr and WC-Co coatings displayed similar value of ηIT, whereas the (Ti,Mo) (C,N)-NiCo coating displayed ηIT values of about 13% lower. As the ηIT value is characterizing the materials elastic-plastic response to external load, the same can be used to estimate the ability of the material to resist abrasive or wear damage.
Scratch Test Procedure and Results
An Anton Paar Revetest Scratch Tester with load range of 1 N to 200 N was used to perform scratch tests. The tests were carried out on the cross-sectioned samples embedded in LECO resin. The scratch tests were performed by Anton Paar automatic Map by Stage procedure using constant loads of 5 N, 29 N, 52 N, 76 N and 100 N. 1.2 mm was the length of the scratch and scratching speed of 2.4 mm/minute and Rockwell C diamond indenter of 200 µm radius were used for all the tests. An optical microscope integrated on the Revetest instrument was used to take images of the cone fracture area, shown in Figure 6. Among the Lx, Ly and cone angle values, the projected cone area, Acn = Lx.Ly (Figure 2) was selected as the most characteristic factor because only Acn displayed a monotonic relationship to the scratching load.
Figure 6. Typical example of the scratch track and cone fracture on the HVOF coating (Cr3C2-NiCr, 52N load).
The results show that as the scratching load increased, the projected cone area also increases. This increase was nearly linear for the WC-17Co and Cr3C2-NiCr samples, while for the (Ti,Mo) (C,N)-NiCo sample the increase displayed a more parabolic trend (Figure 7). This suggests the presence of a different failure mechanism of the (Ti,Mo)(C,N)-NiCo coating at high loads when compared to the Cr3C2-NiCr coating: the projected cone area for both these coatings was very similar at loads up to 50 N. At higher loads, the damage in the (Ti,Mo)(C,N)-NiCo coating was found to be larger than in the other tested coatings.
Figure 7. Projected cone area as a function of the constant load scratch force.
All the cone fractures originated in the coating and this confirms that the tests make it possible to characterize the cohesion of the coating. This cohesion of the coating is closely associated with its abrasion and wear resistance because in both cases the failure begins in the coating. This failure occurs when an external force through abrasive/wear particles or a sliding indenter during scratch test is applied. The results thus obtained match with the already measured abrasion resistance of the tested coatings: the lowest abrasion resistance was shown by the (Ti,Mo) (C,N)-NiCo mixture showed, while the highest abrasion resistance was shown by WC-17Co followed by Cr3C2-NiCr.
The results presented above demonstrate that this type of constant load scratch test that can be employed as a fast, efficient means for characterization of cohesion and can even be used to estimate the abrasion or wear resistance of thermal spray coatings.
The above study shows how the instrumented indentation and scratch testing methods can be effectively used for studying macroscopic and microscopic properties of thermal spray coatings. The methods provide a better understanding of the micro and nano scale mechanical properties. The observed evolution of hardness proves that properties of single splats or grains are measured at extremely low loads, while the effect of the more soft and ductile metal matrix is noted with increasing load. A novel parameter was employed for coating characterization: the ratio of elastic to total work of indentation, ηIT. This parameter makes it possible to characterize the elastic-plastic properties of the coating which is closely associated with its resistance to failure.
On the cross-sectioned coating, the constant load scratch testing provides a fast, intuitive and efficient approach for characterization of cohesion and, in some cases, adhesion of the coating. The research work which is currently going on is now attempting to further develop this method to measure the cohesion and its potential extension for adhesion determination.
It is believed that this comprehensive testing program will provide a better understanding of the association between overall and local properties of these perspective materials.
The Authors would like to thank Dr. Šárka Houdková of Škoda Research, Ltd. Czech Republic, and Dr. Radek Enzl of Flame Spray Technologies, Ltd. The Netherlands, for providing the samples and useful discussions in preparing this Applications Bulletin.
1. A. C. Fischer-Cripps: Nanoindentation. Springer Verlag, p. 198.
2. ASTM E 2546 – 07 Standard Practice for Instrumented Indentation Testing.
3. ISO 14577 Metallic materials – Instrumented indentation test for hardness and material parameters.
4. E. Lopez, F. Beltzung, G. Zambelli: Measurement of cohesion and adhesion strengths in alumina coatings produced by plasma spraying. Journal of Materials Science Letters 8, (1989) 346-348.
5. L.-M. Berger, R. Zieris, S. Saaro: Oxidation of HVOF-sprayed hardmetal coatings. Conf. Proc. Int. Thermal Spray Conference ITSC 2005, 2-4 May 2005 Basel, Switzerland. DVS-Verlag 2005, CD (ISBN 3-87155-793-5), pp. 969-976.
This information has been sourced, reviewed and adapted from materials provided by Anton Paar TriTec SA.
For more information on this source, please visit Anton Paar TriTec SA.