Image Credit: Alemnis AG
To investigate materials’ mechanical properties, nanoindentation and micro-compression tests are often used. These properties of materials are interrogated using either load or displacement control, through an indenter tip of known geometry. Traditional methods of testing hardness such as the Vickers or Brinell techniques, are the predecessors of this instrumented indentation principle, which has emerged as the ideal technology in the evaluation of a wide range of sample properties.
Sample properties might include those of creep, stress relaxation, strain rate sensitivity, hardness, slip deformation, elastic modulus, and fracture toughness. However, it is important to note that, with changes in temperature, significant changes in material characteristics take place.
The temperature dependence, in certain cases, may not have significant implications for downstream applications. This is not the case, however, for materials intended for use in thermal processing, refractory applications or intended for use as cutting tool coatings, operating at high speeds and temperatures. In these instances, materials like this must be characterized in simulated in-service temperatures. This can be a difficult endeavor for traditional nanoindentation equipment.
It is important to establish which nanoindenter materials perform the best under high-temperature testing.
Certain Properties of High-Temperature Indenter Materials
Hot hardness testing can be thought of as the ancestor to this method of assessing the mechanical properties of materials at elevated temperatures, thanks to the fact that it was the first method to encounter many of the challenges and quirks associated with the task.
Indeed, this demonstrates many of the unique principles involved with high-temperature indentation. Hot hardness tests demonstrated the significance of recognizing how properties like the highest possible hardness and stiffness may change under test conditions, given that these were the properties leading to the very selection of indenter probes.
At test temperatures, chosen indenter materials must retain their hardness. These temperatures can range from room temperature, to the levels of heat in industrial applications - often up to 600 °C (1112 °F). Materials may soften under test conditions if they have poor coefficients of thermal expansion which exhibit exceptional hardness at room temperature. Of course, this alteration can change the indenter area function, which has a knock-on effect to the hardness or elastic modulus which is subsequently calculated. An additional factor to consider is the potential permanent damage to the indenter tip, which may lose its sharpness.
In addition, it is key to assess mismatch in chemical reactivity between the sample and the indenter tip, at high temperatures. If indenter materials are inert at low temperatures, they may then react with the sample material under test conditions. Such a reaction would risk causing contamination of either one or both of the materials in contact; and in the most extreme cases, risk the dissolution of one material into the other.
Assessing High-Temperature Indenter Materials
The indenter material must be a minimum of 20% higher hardness than the sample so that plastic deformation can be generated, when performing a hardness test. In ideal circumstances, the indenter should be > 100% harder. This is ideal so that rapid blunting or failure can be avoided.
Wheeler and Michler have compiled a comprehensive analysis of high temperature indenter materials (Ref. 1) which is shown in the hardness vs. temperature plot below. Despite the fact that diamond is naturally one of the best indenter materials at ambient temperatures, its famed hardness undergoes a rapid decline at elevated temperatures, though the material still remains harder than all other candidates.
At all temperatures, diamond is the hardest engineering material available: with analysis available from room temperature up to 1000 °C (1832 °F). Diamond can retain its exceptional intersection of mechanical properties, in the duration of highly stringent high-temperature nanoindentation tests.
It therefore retains the title of the ideal solution for testing extremely hard materials, including technical-grade ceramics and functional coatings. The next most significant indenter material, in terms of hardness at elevated temperatures, is cubic boron nitride (BN): this material retains its rigid structure up to temperatures of approximately 500 °C (932 °F) – after which it undergoes a significant drop in hardness.
Hot Vickers and Knoop hardness of indenter materials as a function of temperature with extrapolated Vickers hardness shown as dotted lines for materials where only Knoop hardness data were available (from Ref. 1). Image Credit: Alemnis AG
Despite the fact that it is not quite as hard as cubic boron nitride, tungsten carbide (WC) outperforms BN when retaining its hardness at high temperatures. Tungsten carbide retains this until a maximum peak operating temperature, of approximately 900 °C (1652 °F).
Of course, it is important to remember that these material properties are simply one aspect of indenter performance. Indeed, at elevated temperatures, the risk of cross-contamination between the sample, the indenter tip and the atmosphere’s oxygen molecules in the atmosphere is significantly greater. When heated to temperatures exceeding 400 °C (752 °F), oxide formation on indenter tips is an almost unavoidable risk for diamond, boron nitride, and sapphire (Al2O3). One of the least reactive indenter materials, at high temperatures, is tungsten carbide, which facilitates its application in the process of high-temperature nanoindentation testing of metal alloys.
It is essential to have an understanding of the material under test before the material of the indenter tip can be diagnosed. Despite the fact that diamond and tungsten carbide seem to be the ideal frontrunners regarding low reactivity and maintained mechanical properties, there will always be exceptions to disprove the rule.
An additional issue is posed by reactivity, in the sense that it may force a temperature limitation on many nanoindentation apparatuses – for instance, with an average maximum operating temperature of 200 °C (392 °F) in air. This limitation may be adequate for proprietary tests, but will prevent accurate insights being provided into the material hardness of high-performance alloys and refractories. One conventional solution to this issue is to conduct heated indentation tests in vacuum conditions.
High-Temperature Nanoindentation Testing with Alemnis
Alemnis offers a wide range of High Temperature (HT) modules ideal for the precision testing of materials where both the sample and indenter can be heated up to 1000 °C (1832 °F). Such modules are retrofittable and compatible with all standard indenter tip materials, which can be rapidly and efficiently exchanged between experiments.
References and Further Reading
- J. M. Wheeler and J. Michler, Indenter materials for high temperature nanoindentation, Review of Scientific Instruments 84 (2013) 101301
This information has been sourced, reviewed and adapted from materials provided by Alemnis AG.
For more information on this source, please visit Alemnis AG.