Recently, I was helping my son put his bed back together – a labor-intensive process involving fleece blankets. He has something like eight or ten of them, all of which he insists on sleeping under when we put him to bed, so most of them end up on the floor because he gets hot and kicks them off. No big deal, really. It’s hardly the worst sleeping habit he could have.
“Now, the big one next,” I tell him, spreading his quilt on the bed.
“Yes,” he agrees. “It’ll make my bed [i]so soft[/i]!” Then he thinks about that as he hands me his Darth Vader fleece blanket. “What’s the softest thing ever?”
You know what? I have no idea.
Hardness versus Softness
“Hard” and “soft” are words that we use with some regularity. Most of us have an intuitive understanding of the meaning, even if we can’t define it with precision. But, in order to further our discussion of the subject, we should get a formal definition of each. Or, at least, that sounds reasonable. The problem is, Dictionary.com has 41 different definitions for the word hard and 38 for soft. The first definition of “hard” is “not soft; solid and firm to the touch; unyielding to pressure and impenetrable or almost impenetrable.” By comparison, “soft” is defined as “yielding readily to touch or pressure; easily penetrated, divided, or changed in shape; not hard or stiff.”
So, by definition, soft things are not hard and hard things are not soft. Thank you, Dictionary.com. Fortunately, the University of Maryland provides a more technical definition of hardness:
The Metals Handbook defines hardness as “Resistance of metal to plastic deformation, usually by indentation. However, the term may also refer to stiffness or temper, or to resistance to scratching, abrasion, or cutting. It is the property of a metal, which gives it the ability to resist being permanently, deformed (bent, broken, or have its shape changed), when a load is applied. The greater the hardness of the metal, the greater resistance it has to deformation.
In mineralogy the property of matter commonly described as the resistance of a substance to being scratched by another substance. In metallurgy hardness is defined as the ability of a material to resist plastic deformation.
The dictionary of Metallurgy defines the indentation hardness as the resistance of a material to indentation. This is the usual type of hardness test, in which a pointed or rounded indenter is pressed into a surface under a substantially static load.
So, in brief, hardness is the ability of a substance to resist being permanently deformed or scratched.
How Do You Measure Hardness?
Sometimes, you just have to turn to Wikipedia: “There are three main types of hardness measurements: scratch, indentation, and rebound. Within each of these classes of measurement there are individual measurement scales. For practical reasons conversion tables are used to convert between one scale and another.”
Scratch hardness is generally measured with a sclerometer, a device that measures “the width of a scratch made by a diamond under a fixed load, and drawn across the face of the specimen under fixed conditions”.
Indentation hardness is primarily used in engineering and metallurgy, and measures “the resistance of a sample to material deformation due to a constant compression load from a sharp object”. Regardless of the specific test used (and there are several), the test uses a dense object of spherical or conical shape, pushed into the material under a specific pressure for a specified period of time. The depth of the resulting indentation is then used to determine how hard the material is.
Rebound hardness “measures the height of the “bounce” of a diamond-tipped hammer dropped from a fixed height onto a material”. The actual device with the hammer is called a scleroscope.
The Softest Thing Ever? How About The Hardest?
Since there isn’t a standardized definition of hard or soft, can we actually talk about the “hardest’ or “softest” thing? The answer is “sort of”. For example, diamonds are able to survive pressures of around 150 GPa. GPa is the scientific notation for “gigapascals”, where one pascal is helpfully defined as the pressure exerted by a force of magnitude one newton perpendicularly upon an area of one square meter (or, if this makes more sense, one kilogram of mass over one square meter for one second). For comparison purposes, one atmosphere of pressure is 101.325 kilopascal (kPa) – which is .000101325 GPa.
In other words, natural diamonds can withstand approximately 1,480,000 times the pressure of Earth’s atmosphere at sea level. Impressive, right? Well, yes. Yes it is. But researchers in the Technological institute for Superhard and Novel Carbon Materials and the Moscow Institute of Physics and Technology have developed a new method of synthesizing ultrahard fullerines, which can withstand upwards of 300 GPa – although the process is not yet ready for industrial scales. Why? Well, making the ultrahard fullerite requires pressures in excess of 13 GPa – the article states that “modern equipment cannot provide such pressure on a large scale.”
Looking at a different measure of hardness, neutron degenerate matter (aka the stuff neutron stars are made out of) is theorized to have a Young modulus that is 20 orders of magnitude larger than that of diamond. So, by at least one measure of hardness, that is the hardest thing known.
Softness is trickier. The softest possible thing would be a material with no resistance to deformation or scratching. You could argue that a vacuum has none of those resistances, but then you could also argue that vacuum is – by definition – not a material. Maybe whipped cream, or soap suds? Honestly, just like you get arguments about how hard something is based on the test applied, you’d probably get similar arguments for how soft something is.
So I’m declaring kittens the softest thing ever. Try to prove me wrong.