Why humidity matters
Water is a plasticiser for many materials. Nylons, polyurethanes, hydrogels, cellulose films, biological tissues, pharmaceutical tablets and many coatings absorb moisture from the air and become softer and more prone to creep as they do. A hardness measurement at 30% RH and another at 70% RH can describe very different materials. For products used in changing climates, or in the body, mechanical properties need to be known as a function of humidity.
How the module works
A chamber encloses both the indenter and the sample, and the humidity controller sets the relative humidity anywhere from below 5% up to the dew point. Because tip and sample are inside the same uniform environment, the indentation contact itself is at the set humidity. Samples are allowed to equilibrate at each set point before testing, since moisture uptake takes time.
What the data show
In NANOVEA’s humidity-controlled study on a PB1000, a polymer film was indented with a Berkovich tip at 10 mN at 25, 35, 45, 55, 65 and 75% RH, with hardness calculated per ASTM E2546 and the Oliver & Pharr method:
- from 25% to 55% RH, hardness decreased gradually from about 0.60 to 0.54 GPa, and creep depth rose from 36 to 48 nm;
- at 65% and 75% RH, hardness dropped to 0.46 and 0.31 GPa, and creep depth climbed to 80 and 105 nm.
The non-linear response, gradual and then rapid, is typical of moisture uptake, and a single room-air measurement would have missed it entirely.
Combining humidity with other tests
Creep, nanoindentation DMA, scratch and low-load friction can all run in the humidity chamber. For friction and wear at controlled humidity under tribological loads, NANOVEA tribometers offer a corresponding humidity and inert-gas module.
