High Temperature Indentation Testing
Materials properties are more or less affected by the environmental temperature change. Nowadays more and more devices are operated at elevated temperatures, ranging from consumer electronics like cellphones, laptops, and gaming systems, to the kitchen wares like cooking pans, ovens, and microwaves, and to car engines, drill bits, and aircraft materials. Understanding the materials properties change with temperature rising is one of the most important yet largely overlooked fields in surface mechanical analysis.
To achieve high temperature, a heating stage or chamber is usually added to the instrumented indentation system. The specimen is put on the stage for heating while the indenter tip is also heated but separately. Once the required temperature is reached (measured by three thermal couples, one on stage, one on sample surface, and the last on the indenter tip), the indentation process starts just like normal nano or micro indentation. Environment puts a huge influence on the high temp mechanical measurements. The heating stage setup is usually ideal if the maximum temperature does not exceed 500 °C. If the temperature goes beyond 500 °C, both the specimen and the indenter tip start to oxidize rather quickly. In this case, a vacuum and gas-purging chamber is required to keep the environment steady.
Heating stage add-on to nanoindenter system
The biggest challenge for high temperature nanoindentation is the thermal drift. Most of the materials in the world will expand when heated. Imagine the popcorn bursts into a large bulk from a small corn seed when heated in the microwave. Most of the solid materials won't increase its size drastically like that, but the degree of expansion still exists. The so-called thermal expansion coefficient is used to quantify the expansion degree of materials. Due to atomic and crystalline structure differences, most of the materials have different thermal expansion coefficients (degree). The nanoindenter systems are made from different materials (e.g. the stage is steel, tip is diamond, etc.). When heated the system materials expand with various degrees. Such expansion mismatch causes a tiny drift in the Z direction (usually in 1 to 100 nm/s), where the nanoindentation is performed. Such a thermal drift means nothing to macro scale testing. However, during a nanoindentation test, the final indent depth is usually less than 1000 nm, sometimes less than 100 nm. The 1~100 nm/s thermal drift now becomes a serious issue that causes a huge error in nanoindentation depth measurement. Obviously, the thermal drift issue worsens with rising temperatures. Thus eliminating the thermal drift is one of the most important tasks for high temperature nanoindentation.
Another serious issue of high temperature nanoindentation is that the high precision sensors used on the nanoindenter systems have limited range of working temperatures. The high temperature around the sample and the indenter during test might also cause these sensors to overheat, which generates error and noise in the measurements. Thus how to insulate heat from the sensors is also an important task to achieve for high temp indentation.
One of the best ways to reduce thermal drift during high temp nanoindentation is to heat the indenter tip separately. If the indenter is not heated to the same temperature that the sample possess, as soon as the indenter touches the sample surface, the heat is immediately transferred to the indenter which breaks the thermal equilibrium and causes drift and noise in the data. To heat the indenter, a separate and small heating element is attached to the diamond tip, while an additional thermal couple is used to measure the surface temperature of the indenter. Once both sample and indenter tip reach the same temperature and stabilize, the test starts.
As mentioned, the nanoindentation sensors should be insulated from heating during tests. The unique pendulum design from MicoMaterials Nanotest system nicely separates the sensors from the indenter tip and the sample, which makes it most ideal for high temperature nanoindentation. Other methods like thermal barrier layer and liquid cooling circulation are also useful to reduce the sensor overheating.
Although air is not a good heat-transfer media, if high temp indentation is performed on an open-frame system, it still causes thermal gradient and the temperature equilibrium is difficult to reach. Thus introducing a small enclosure around the indenter and sample is much preferred for high temp tests.
Last but not least, the vacuum and gas purging chamber works nicely if the testing temperature is extremely high (over 500 °C) to prevent sample and indenter tip from oxidization.
1. Glass Transition Point
2. Modulus/Hardness at high temp
3. Creep/Relaxation degree change
5. Fracture Strength at high temp
ASTM standard E2546 and ISO standard 14577 for instrumented indentation can still be used as the general guidline, however, certain parameters need to be modified in order to better suit the high temperature testing conditions. For instance, ASTM E2546 indicates a 30 sec loading/unloading process which needs to be shortened to just a few seconds during a high temp creep test to make sure all the creep degree is captured at the constant load holding period.