Characterization of Plasma Sprayed Coatings by Indentation

In this application note, the benefits of using instrumented indentation method on plasma sprayed coatings relating to their heterogeneous microstructure will be discussed in detail.

The paper will also focus on the application of the instrumented indentation technique to plasma sprayed metals and ceramics, and the use of scratch testing for characterization of HVOF coatings.

Instrumented Indentation Method

When compared to traditional Vickers or Knoop microhardness methods, instrumented indentation method provides more data about a thermal spray coating.

Although the force range tends to overlap, the results obtained through instrumented indentation offer more information about the coatings’ mechanical properties, such as creep or cyclic behavior, plastic/elastic work of indentation, and elastic modulus.

Loading-unloading indentation curve presenting two 'indenter drops' due to indentation on voids or other defects.

Figure 1. Loading-unloading indentation curve presenting two 'indenter drops' due to indentation on voids or other defects.

The instrumented indentation approach has a force-displacement recording capability and hence able to sense when an indentation is conducted on a void. Such effects may not be observed in traditional Vickers measurements.

In addition, instrumented indentation offers automated measurements owing to the hardness and calculation of elastic modulus from the loading-unloading curve. The automation can be categorized into two types, such as automated indentation load increase and spatial automation (matrix indentation).

The latter is particularly useful for heterogeneous thermal spray coatings. The majority of the results presented in this paper were acquired through the Visual matrix indentation technique.

Continuous Multi-cycle Indentation

Continuous multi-cycle indentation is an efficient technique, which enables users to automatically raise the maximum load on each following cycle. Depending on the number of cycles, information on hardness, indentation depth or elastic modulus is achieved within a short period of time.

This technique is useful for examining elastic modulus and hardness of thermal spray coatings at different depths and loads.

Indentation Parameters

For characterization of the coating’s mechanical properties, two CSM Instruments systems such as Indentation Tester (MHT) and Nanoindentation Tester (NHT) were used. The latter is a high-performance nanoindentation system that includes a Berkovich indenter, an optical microscope and motorized tables. It has a load range of 0.1 mN to 500 mN. The MHT has a load range of 0.03 N to 10 N and is equipped with a Vickers indenter.

This instrument was positioned on a Compact Platform integrated with optical and confocal microscopes. Using the CMC method, measurements were performed and the indented areas were chosen by Visual matrix in majority of cases. On each sample, about five to seven measurements of the same type were carried out and average values were obtained.

Plasma Sprayed Alumina

The water stabilized plasma (WSP) spraying is a special method used for spraying metals and ceramics. This technique has been utilized for a number of years for spraying chromium oxide, alumina and steel coatings.

The main benefit provided by this spraying technique is high enthalpy of the plasma which enables spraying of materials with high throughput and high melting point. The spraying and feeding distances are important spraying parameters since they measure the material’s mechanical properties.

Indentation depth (nm)

Figure 2. Indentation depth (nm)

Hardness (a) and elastic modulus (b) as a function of indentation depth measured on cross-section of alumina coatings.

Figure 3. Hardness (a) and elastic modulus (b) as a function of indentation depth measured on cross-section of alumina coatings.

Plasma Sprayed Stainless Steel

In order to represent a different type of materials to the plasma sprayed ceramics, stainless steel coatings were chosen. The steel coatings were sprayed by utilizing the WSP torch from a 316L stainless steel powder onto a steel substrate.

The coatings’ thickness was ~300 μm, apart from the sample 1412 whose thickness was 900 μm. Then, the samples were sprayed under different conditions in order to obtain different fraction of oxides, which are usual for metallic coatings sprayed in air.

Using the NHT instrument and the CMC method, the indentation was done at loads from 1 mN to 100 mN with 20 cycles. Moreover, preferred homogeneous areas were tested by single load indentations at 10 mN. A Berkovich indenter was used for all indentations.

Precise location of indentations on WSP sprayed steel; arrows show the indents.

Figure 4. Precise location of indentations on WSP sprayed steel; arrows show the indents.

Conclusion

The instrumented indentation method is especially useful on heterogeneous materials and enables a comprehensive study of separate coating constituents and the overall properties of coatings which was not possible in the past.

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.

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