Indentation Hardness

Among the three hardness types shared in the previous article, indentation hardness is the one which most engineers are familiar with. In fact, I bet that the general reply to the definition of hardness matches that of indentation hardness so much so that they are interchangeable in most discussions. Even engineering textbooks seldom mention indentation hardness as a separate topic – it is instead discussed as a hardness topic without an in-depth subtopic on other hardness types. 

Ball indenter used for Rockwell hardness testing. Different macroindentation test methods uses different indenter geometry and measures different dimensions of the indentation geometry (image taken from Instron).

Indentation hardness quantifies the resistance of a sample to material deformation, plastic deformation to be specific, due to the effect of a constant compressive load of a sharp object, hence the term “indentation”. In this test, a specific indenter is exerted onto the test material until an indentation is formed, of which dimension is measured and determines the hardness in accordance to a certain scale. 

For engineers personally involved in testing materials, it would be important to understand that most indentation tests have specific load rate and specific dwell time (the period in which the indenter load is held without moving), but not all of them are so. 

Hardness - How Hard Is Hard?

It is quite easy to say that a piece of metal is harder than a block of rubber, but how hard is hard? And is a cast iron harder than a stainless steel sheet? How would one find out information about hardness when you are reading the material properties from a specification sheet? 

In fact, when I read more into this property, it is not as simple as we usually would have thought, seeing that we use the descriptive term daily in our conversation. It could be said that: 

Hardness is a measure of how resistant solid matter is to various kinds of permanent shape change when a force is applied.

Diamond - not only were they a fascination with the ladies, they were also a fascination of engineers. With its reputation of being the hardest substrate ever known, it could cut through almost everything else. So how would you define the hardness of a diamond? (image taken from Jonathan's Fine Jewelers)

Gloss - Measurement of Gloss Part 1

As was established in the previous post, gloss is determined by the amount of light rays being reflected under specular reflection as opposed to diffuse reflection. The more rays reflected through the former method, the glossier the surface is. 

Taktik iPhone case designed by Minimal studio. The surface is described as glossy and although this image is (I suspect) a rendered image, the glossiness of the surface is still conveyed. For quality control, determining the item as meeting the expected gloss level is not acceptable through mere visual inspection (image taken from DesignBoom).

Gloss could be compared qualitatively by comparing reflection of an image off different surfaces. Surface exhibiting sharper reflection will be considered glossier. But this is only useful in a qualitative way, and only if both surface reflections are distinctly different. Different observant will have different sight and recognition capability as well, so this method is physiologically affected. This qualitative method will not suffice as a quality control method.

Gloss - A Perception of Surfaces Part 2

Unlike other physical measurements, gloss measurement is not as straightforward. But it isn't difficult either, once one understood how gloss is perceived and interpreted by the human eye.

Gloss is an aspect of visual perception of objects, and it is an attribute of surfaces. With this in mind, any material could be made to meet the requirement of any gloss level as long as the technology exists and the method economic. 

What in this picture makes you think that the car's exterior is "glossy"? What causes this term to come to mind? (image taken from Ziebart)

Gloss - A Perception of Surfaces Part 1

You take a look at the shiny metallic surface of a sleek sports car and you immediately thought of it as "glossy". You saw another car with a body which reflects none of the ambient lighting, absorbing everything like a black hole, and you think of it as "matte".

The glossy surface of an Audi TT (image taken from DeviantArt).

As easy as the word comes to your mind, how does one define "glossy"? This would be important to an engineer, since there has to be some way to control the quality of a surface, and control will not be possible without measurement, and no measurement without a solid definition.