Significance and Use
This test is intended to assess the mechanical integrity, failure modes, and practical adhesion strength of a specific hard ceramic coating on a given metal or ceramic substrate. The test method does not measure the fundamental “adhesion strength” of the bond between the coating and the substrate. Rather, the test method gives a quantitative engineering measurement of the practical (extrinsic) adhesion strength and damage resistance of the coating-substrate system as a function of applied normal force. The adhesion strength and damage modes depend on the complex interaction of the coating/substrate properties (hardness, fracture strength, modulus of elasticity, damage mechanisms, microstructure, flaw population, surface roughness, and so forth) and the test parameters (stylus properties and geometry, loading rate, displacement rate, and so forth).
The quantitative coating adhesion scratch test is a simple, practical, and rapid test. However, reliable and reproducible test results require careful control of the test system configuration and testing parameters, detailed analysis of the coating damage features, and appropriate characterization of the properties and morphology of the coating and the substrate of the test specimens.
The coating adhesion test has direct application across the full range of coating development, engineering, and production efforts. Measurements of the damage mechanisms in a coating as a function of applied normal forces are useful to understand material-process-property relations; quantify and qualify the mechanical response of coating-substrate systems; assess coating durability; measure production quality; and support failure analysis.
This test method is applicable to a wide range of hard ceramic coating compositions
Note 2—Under narrow circumstances, the test may be used for ceramic coatings on polymer substrates with due consideration of the differences in elastic modulus, ductility, and strength between the two types of materials. Commonly, the low comparative modulus of the polymer substrate means that the ceramic coating will generally tend to fail in bending (through-thickness adhesive failure) before cohesive failure in the coating itself.
Ceramic coatings can be crystalline or amorphous, but commonly have high relative density with limited porosity (<5 %). Porous coatings can be tested, but the effects of porosity on the damage mechanisms in the coating must be carefully considered.
The test method, as defined with the 200 μm radius Rockwell diamond stylus, is commonly used for ceramic coating thicknesses in the range of 0.10 to 30 μm. Thinner coatings may require a smaller diameter stylus and lower normal forces for reliable results. Thicker coatings may require larger diameter stylus and higher normal forces. Any variations in stylus size and geometry and designated normal force ranges shall be reported.
Specimens commonly have a flat planar surface for testing, but cylinder geometries can also be tested if they are properly fixtured and aligned and the scratch direction is along the long axis of the specimen. The physical size of the test specimen is determined primarily by the capabilities and limits of the test equipment stage and fixturing.
The test is commonly conducted under unlubricated conditions and at room temperature. However, it is feasible and possible to modify the test equipment and test conditions to conduct the test with lubrication or at elevated temperatures.
Coated specimens can be tested after high temperature, oxidative, or corrosive exposure to assess the retained properties and durability (short-term and long-term) of the coating. Any specimen conditioning or environmental exposure shall be fully documented in the test report, describing in detail the exposure conditions (temperature, atmosphere, pressures, chemistry, humidity, and so forth), the length of time, and resulting changes in coating morphology, composition, and microstructure.
The test method as described herein is not appropriate for polymer coatings, ductile metal coatings, very thin (<0.1 μm) ceramic coatings, or very thick (>30 μm) ceramic coatings.
Scope
1.1 This test method covers the determination of the practical adhesion strength and mechanical failure modes of hard (Vickers Hardness HV = 5 GPa or higher), thin (≤30 μm) ceramic coatings on metal and ceramic substrates at ambient temperatures. These ceramic coatings are commonly used for wear/abrasion resistance, oxidation protection, and functional (optical, magnetic, electronic, biological) performance improvement.
1.2 In the test method, a diamond stylus of defined geometry (Rockwell C, a conical diamond indenter with an included angle of 120° and a spherical tip radius of 200 μm) is drawn across the flat surface of a coated test specimen at a constant speed and a defined normal force (constant or progressively increasing) for a defined distance. The damage along the scratch track is microscopically assessed as a function of the applied force. Specific levels of progressive damage are associated with increasing normal stylus forces. The force level(s) which produce a specific type/level of damage in the coating are defined as a critical scratch load(s). The test method also describes the use of tangential force and acoustic emission signals as secondary test data to identify different coating damage levels.
1.3 Applicability to Coatings—This test method is applicable to a wide range of hard ceramic coating compositions: carbides, nitrides, oxides, diamond, and diamond-like carbon on ceramic and metal substrates. The test method, as defined with the 200 μm radius diamond stylus, is commonly used for coating thicknesses in the range of 0.1 to 30 μm. Test specimens generally have a planar surface for testing, but cylinder geometries can also be tested with an appropriate fixture.
1.4 Principal Limitations:
1.4.1 The test method does not measure the fundamental adhesion strength of the bond between the coating and the substrate. Rather, the test method gives an engineering measurement of the practical (extrinsic) adhesion strength of a coating-substrate system, which depends on the complex interaction of the test parameters (stylus properties and geometry, loading rate, displacement rate, and so forth) and the coating/substrate properties (hardness, fracture strength, modulus of elasticity, damage mechanisms, microstructure, flaw population, surface roughness, and so forth).
1.4.2 The defined test method is not directly applicable to metal or polymeric coatings which fail in a ductile, plastic manner, because plastic deformation mechanisms are very different than the brittle damage modes and features observed in hard ceramic coatings. The test method may be applicable to hard metal coatings which fail in a brittle mode with appropriate changes in test parameters and damage analysis procedures and criteria.
1.4.3 The test method, as defined with the Rockwell C diamond stylus and specific normal force and rate parameters, is not recommended for very thin (<0.1 μm) or thicker coatings (>30 μm). Such coatings may require different stylus geometries, loading rates, and ranges of applied normal force for usable, accurate, repeatable results.
1.4.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard. Test data values in SI units (newtons (N) for force and millimetres (mm) for displacement) are to be considered as standard and are in accordance with .
1.4.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
