Glass Hardness Question

  1. I am currently working on a research project aimed at explaining hardness variation of glasses with applied load (indentation size effect) and loading time (hardness creep). I am using the Vickers Hardness scale.

    My question (that hopefully someone here can answer) is what factors and properties of a material affect its hardness? In glasses, there is a correlation between fictive temperature of a glass and hardness, but I'm looking more for atomic scale properties that influence hardness.

    Any insight would be greatly appreciated.
  2. jcsd
  3. Gokul43201

    Gokul43201 11,141
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    Simply put : it's the microstructure of the surface (and near-surface material). Some factors (like distribution of particles, cracks and grain orientations) which affect the hardness in a regular macroscopic hardness test will play different roles in micro- or nano-hardness tests. Since indentation hardness is strongly correlated with the yield strength for many materials, factors which affect the latter will also affect the former. I'm not familiar with hardness testing of glasses (I assume this refers to silicate glasses ?), but I imagine that modelling the mechanical properties of a glass microscopically would be harder than for a regular crystalline solid. There may be non-equilibrium behavior (creep) to address as well.

    Perennial ?
  4. Yes the glasses I am studying are silicates (both Soda-Lime and Pure Silica).

    Creep is certainly existent in the samples as you pointed out. That hardness is correlated with yield strength is helpful.

  5. PerennialII

    PerennialII 1,102
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    Tough one ... I can't really add that much to Gokul's explanation about the microstructural and related basic characteristics, the glass type and its microstructure being the most important characteristic - defect, particle etc. distributions (and all sorts of microstructural variations and gradients) and differences within them being the differentiating factor when comparing within certain types of glass and so forth.

    One of the "problems" is likely that since we're considering something which behaves in a brittle sense, stress - measures are difficult to apply (if you think about hardness testing of metals for one .... yield and tensile strengths can be directly correlated to hardness) and what you're left with are the Young's modulus and fracture toughness (or a measure of fracture strength, tensile strength .... what you got under there is deformation (=linear-elastic + your time dependent one) and fracture (=linear-elastic fracture mechanics) -> essentially if you'd do a fracture analysis of the system you'd have all the parameters and elements which influence included). I would also consider residual stresses important, quite often in coatings etc. brittle systems they can vary (I don't have any glass experience, I'm extrapolating what I know & have done with ceramics overall) significantly and when comparing different types can be the deciding factor. Would also consider material property gradients in thickness direction and overall variation of all above in thickness direction, can arise easily from manufacturing etc.

    Creep .... fashinating .... wouldn't have thought this at first, should investigate what sort of viscoplastic response glass can have ... if it can be characterized with something like a power-law strain-hardening creep models or so the parameters of those material models are natural to be used in the analysis (like constants of a basic Norton's law). Haven't seen anyone do any creep evaluation in this respect ... interesting ... can likely evaluate the overall significance when have some material data applicable for this case.

    If you like I can provide you references and stuff about modeling indentation (I'm involved in one project where we're doing similar stuff for coatings).
  6. Yeah, any references are always great.

    The hardness creep is a curious feature that we have been investigating. Basically, there is a logarithmic decay of hardness with time. A similar type of hardness decay with varying indentation loads is also seen (termed indentation size effect that you may be familiar with).

    As far as I know, not a whole lot of work has been done in investigating these phenomena. There are a few highly empirical explanations (Meyer's Law, modified Meyer's Law among them). If you're interested, I'd be glad to point out some journal entries that are related to hardness creep and indentation size effect in glasses.

    Thanks for the responses!
  7. PerennialII

    PerennialII 1,102
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    Yep, the indentation size effect is the one we're working on & I'm more familiar with .... since the problem we're working on is essentially a 'hard' coating (TiN, DLC MoS2) on a soft substrate at room temperature what we've is a size effect arising from the failure & cracking of the coating and elastoplasticity of the substrate time - independently.

    ... since I'm essentially doing the fracture modeling I can PM/mail you stuff about analyzing & modeling indentation (typically FE stuff combined with modeling efforts & experimental work etc.) of various systems ... this seems to be one of those things which is getting ever more attention, articles have started potting about introducing new models, improvements etc. of the classic approaches. I'd be interested in catching on about the hardness creep, we've some work coming about where references would be handy (nanoindentation & micro - scratch-testing involving also glass).
  8. Sure, anything you could PM or e-mail would be great. I'm always glad to find out about new studies and research.

    If you wanted to find out a little about the hardness creep, there is a good article in the Journal of the American Ceramic Society 70 [7] 497 (1987). I'm not sure if journals like this are readily available to you (since I work in the University setting, I'm used to all this stuff being at the campus library). If it is difficult for you to get your hands on, let me know and I'll locate an e-copy of it for you.

    The hardness creep is thought to possibly be the result of hydroxyl adsorption. Supporting evidence for this is that in nonaqueous liquids (in my work, principally toluene) the hardness creep is not observed. This is the most popular and coherent theory to my knowledge.
  9. PerennialII

    PerennialII 1,102
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    ... I'll get back to you on that, have collected bunch of stuff down the road (pdfs).

    I'll look that article up and see what it holds, I'm enjoying a similar subscription.
  10. PerennialII

    PerennialII 1,102
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    .... just read the article from J. Am. Ceram. Soc. ... the interpretation of the time-dependency using the surface and volumetric energy contributions was an interesting one, gave new insight to the problem and overall to the difficulties in characterizing glass in the environments they worked on ... and considering the time - dependency to arise from a bulk structural transformation in the presence of water (time-dependently), interesting stuff. Wouldn't have thought water having such an effect in decreasing the yield strength of glass, but always seem to come across new stuff :smile: . Certainly an environmental effect I've to see, and they aren't able to offer a very complete mechanistical explatation at least in this paper or really build up a model to approach the problem via for example a constitutive analysis (but make a good case that they've identified the causal factors right - would be interesting to approach this by introducing the experimentally noted effects to a time-dependent material model for glass having a sort of a "concentration" dependency and seeing where it goes from there - someone has probably given this some serious thought?), would think you've some interesting times ahead.
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