Editorial Feature

Hardness Testing of Ceramic Coatings

Ceramic coatings are used in motorsports to protect against heat and wear.

Ceramic coatings are used in motorsports to protect against heat and wear. Image Credits: Marcin Krzyzaks/shutterstock.com

Ceramic coatings are used in a wide variety of industrial and commercial manufacturing processes, including within the automotive industry, in manufacturing processes, and in the design of industrial parts. These coatings are often used to make mechanical components harder, more resistant to corrosion (i.e. rust), impervious to liquids and gases, better insulated (both electrically and thermally), and overall more resistant to wear.

Selecting a Ceramic Coating

The type of ceramic coating and appropriate method of deposition depends on various factors:

  • the desired thickness of the coating
  • the desired function of the ceramic coating
  • the expectations of the component’s operation
  • various economic considerations
  • the substrate material
  • the size and shape of the area to be coated

Thin coatings, such as those used on cutting tools, are most often applied through a vapor deposition process, either chemical (CVD) or physical (PVD). Thick coatings, such as those used in decorative enamels, are commonly applied by a thermal spraying and enameling process.

Types of ceramic coatings include titanium nitride and chromium carbide. These coatings can be applied to steel and other metal parts to increase resistance to abrasion, and to increase a material’s ability to withstand extremely high temperatures.

The Purpose of Hardness Testing for Ceramic Coatings

The mechanical property of a material’s hardness is defined as a specific and calculable measure of how resistant a material is to compressive force. Hardness is a representation of a material’s resistance to localised plastic deformation.

Strong intermolecular bonds generally produce a high level of hardness in a material. The measurement is dependent on the measured elasticity, ductility, stiffness, plasticity, strain, strength, viscosity, and toughness of the material being evaluated for hardness.

Applications of hardness testing of ceramics include defining the hardness gradient of a material, the surface hardness, the case depth, coating hardness, phase hardness, grain hardness, hardness at grain boundaries, and hardness of powders.

The hardness of a ceramic is defined by its chemical composition, including porosity, grain size, and grain-boundary phases. Ceramic hardness is usually tested using either the Vickers or Knoop method, most often using diamond indenters, the processes of which are defined in further detail below.

The Methods and Process of Hardness Testing

There are multiple measurements of hardness, including scratch hardness, indentation hardness, and rebound hardness. Each type of measurement is based on an individual measurement scale, however, conversion between scales is possible for practical purposes.

Indentation tests are a common method of testing the hardness of a ceramic material. Indentation is a straight forward test of penetrating a given material with an indenter under a pre-defined indentation load, then measuring the resulting indentation.

Indenters come in a variety of different shapes and sizes, and the load can be set for nano, micro, or macro indentation ranges, so as to specify the range of mechanical properties that will be tested.

While direct comparison across the various methods of hardness testing is not always possible, the general concept behind the measurement is very similar: the harder the testing material is, the smaller the indentation will be.

Dense ceramics are often measured for hardness by the Knoop hardness test, a method of microindentation optimized for brittle materials or thin coatings such as ceramic. The Knoop hardness test is most practical for the purpose of ceramic coating tests, as only a small indentation is required to evaluate and measure a material’s hardness. Other methods, such as the Vickers hardness test and Rockwell scales, can also be used to determine hardness, but are known to cause more damage to the testing material than the Knoop method.

In the Knoop test, a diamond indenter shaped like a long pyramid is used on a polished surface under a predetermined load for a predetermined length of time (an example of both is a 500 g load held for 10 seconds). The indention is then measured under a microscope and the Knoop hardness (HK) is calculated. The formula used to determine the Knoop hardness of a material is as follows:

HK = load (kgf)/impression area (mm2)

     = P/CpL2

where P equals the load, Cp equals the correction factor related to the shape of the indenter (generally 0.070279), and L equals the length of the indentation along its long axis. The majority of oxide ceramics tested have a Knoop hardness of 1000 to 1500 kgf/mm2.

The Knoop method has a few disadvantages; particularly worth noting is the need to use a microscope to measure the size of the indentation created by the test. The amount of time required to apply the indenter may also be considered a drawback of this test.

The Vickers hardness test method uses a square-based diamond pyramid indenter to penetrate the testing material.

The load, generally weighted between 1 and 120 kgf, is applied for 30 seconds and calculated using the following formula:

HV = 1.854*F/D2

where F is the measurement of the applied load in kg and D is the length of the impression diagonal in mm.

Image Credits: Amnarj Tanongrattana/shutterstock.com

As in the Knoop test, the impression is measured on the diagonal with the assistance of a microscope. The Vickers four-sided indenter is known to crack brittle materials, in which case the Knoop method is likely preferred. It is also worth noting that a direct comparison between Vickers and Knoop hardness numbers is not possible due to the differences in indentation method.

Thinner ceramic coatings are often measured by nanoindentation methods using a Berkovich indenter. The Berkovich tip is a nearly flat, three-sided pyramid with a sharp point used to make indentations to test the hardness of materials greater than 100 nanometers thick.

Similar to the Knoop method, nanoindentation requires the placement of an indenter tip, such as the Berkovich indenter in this case, resulting in the measurement of the indentation created by the added pressure of a defined load. In this scenario, hardness (or H) is equal to the max load (or Pmax) over Ar (or the residual indentation area).

Sources and Further Reading

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