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With a Janka Hardness of 5,060 lbf, the Australian Buloke is considered to be the hardest wood on Earth. Compared to other materials however, it still yields a weaker Compressive Strength and Modulus of Rupture than Steel, and a significantly lower compressive strength than Concrete.

While I could not find data for the Buloke, the Quebracho species is almost as hard, and is only half as strong as steel by these measurements. Quebracho has around 12000 lb/in$^2$ compressive strength and 20000 lb/in$^2$ Modulus of Rupture, to steel's max of well over 100000 lb/in$^2$ for both.

  • What is the hardest breed of wood that can exist, either naturally or through intentional genetic modification or breeding?
  • Where in modern society would such a material be practical based on its physical properties? (Extra consideration for any situation in which it would be the best material for the job)

Notes

  • Physical appearance is in no way a factor as far as this question is concerned.
  • "Hardness," for the purposes of this question, means highest Janka Hardness.
  • Even if the answer to the second part is that it isn't always effective, I'd still like the first part answered.
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    $\begingroup$ What are you trying to do with the wood, that will have a big effect on how to make it stronger. to build large buildings with wood weight is just as important as strength for instance, it is not a coincidence that the largest trees on earth are not make of of the strongest wood, but by a balance of strength on lightness. $\endgroup$ – John Feb 27 '17 at 3:45
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    $\begingroup$ I find this question hard to answer. I used to study architecture, and always liked playing with materials a lot. The aesthetics aside, each material has its pros and cons, and there is no such thing as a general-purpose-best material. It always depends on the application. $\endgroup$ – Burki Feb 27 '17 at 8:25
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    $\begingroup$ What's required here? Ultimate hardness or ultimate durability? Something durable made of wood would have some level of flexibility to absorb stresses without shattering. $\endgroup$ – Snow Feb 27 '17 at 8:51
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    $\begingroup$ Compressive strength and hardness are different things, and both are different to tensile strength. Concrete has good compressive strength; steel has good tensile strength. That's why they're often combined, to get the best properties of both. Hardness is not the same thing; some very hard materials can be surprisingly brittle. Since you seem to be confusing hardness and strength, it would probably be helpful to clarify exactly what properties of wood you're looking for and what applications you're thinking of using them for. $\endgroup$ – Simba Feb 27 '17 at 15:06
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    $\begingroup$ "to steel's max of just over 44000 lb/in2" Where are you getting your steel data?? Good steel can be ballpark 5x (or more stronger.) FYI see: $\endgroup$ – Catalyst Feb 27 '17 at 16:39

12 Answers 12

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Why would someone ever use a 'worse' material to do a specific job?

All engineering projects seek to minimize the use of various resources to achieve the desired results; 'worse' materials are frequently used because cost and availability far outweighs the effectiveness of pure material efficiency.

For example, copper wiring is used in almost all electrical applications. Why? Because it is a good cheap conductor. Is it the best conductor? No; copper's electrical conductivity of $0.596 \cdot 10^6/\text{cm}\ Ω$ is only about 95% of silver's $0.63 \cdot 10^6/\text{cm}\ Ω$. What makes copper more desirable for more applications, its saving grace, is the fact that it costs around $\$2.7/\text{lb}$ (ranging 2-4 \$/lb over the last 5 years) compared to silver's $\$267.8/\text{lb}$ (ranging 200-500 \$/lb over the last 5 years) pricetag. In fact, a number of applications are attempting to convert over to aluminum wiring because aluminum's $0.377 \cdot 10^6/\text{cm}\ Ω$ is still quite reasonable for it's lower cost of $\$0.85/\text{lb}$ (ranging 0.65-1.20 \$/lb over the last 5 years).

Concrete and steel are basically the coppers of compressive strength and tensile strength, respectively. They are used in most skyscrapers and other large construction projects because they are very good at doing their job efficiently at a reasonable price. Wood is typically used for smaller-scale jobs where pure material efficiency is less important than costs associated with greater availability and ease of construction.

Where cost is not the limiting factor, other more specific design criteria may exist. Wood is a dielectric (i.e. non-metal) and so it may be more desireable in applications where radiofrequency reflections would be undesireable. Similarly it is non-magnetic and doesn't become magnetized the way a ferrous metal like steel does. Wood is also relatively lightweight compared to concrete and steel, it can be handy when hulking physical dimensions are of less concern than material density.


What is the hardest breed of wood that can exist?

This question is a little harder to answer outright with numbers, but the sky is the limit if you allow for ultra-futuristic levels of genetic modification.

Wood is so strong relative to its weight because it is essentially a naturally-growing metamaterial. The plant's cells have walls composed of cellulose fibers and linked lignin polymers (which have high tensile and compressive strength, respectively) and form a matrix of repeating boxes which allows for a large amount of rigidity even after much of the cells interior water weight has been dried out. Bioengineering the organism to be stronger would only require designing better organic replacements for cellulose and lignin (and/or optimized versions). Carbon nanotubes or graphene sheets aren't completely out of the realm of possibility here. Additionally, designing more efficient metamaterial structures is another way to improve wood's bulk-material properties. The cellulose and lignin of wood forms a matrix of mostly-rectangluar building blocks, which is essentially a simple cubic crystal lattice formed of extra-cellular cell walls. Engineering plants to have more complex intra-cellular supportive structure could allow for these structures to more closely resemble diamond cubic lattice structures and increase strength by quite a bit.

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  • $\begingroup$ Of course, once you're growing graphene/carbon trees, does it still count as "wood" ? $\endgroup$ – Shadur Feb 28 '17 at 13:52
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    $\begingroup$ If you have nanobots building a framework of carbon nanostructures, then you have more of a nanofactory than a "tree" so there'd be little sense in calling the material "wood". However, if you still have a tree which grows by stacking cell after cell and simply uses nanotubes as an optimally-dengineered-cellulose in its cell walls, then why not call it wood? $\endgroup$ – fenix d.Anconia Feb 28 '17 at 14:19
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    $\begingroup$ Another good example to add for the wiring would be aluminum, which is actually lighter and cheaper than copper for wiring and is used for long overhead wiring for those reasons, but has a number of disadvantages that offset that for many other applications like house wiring and electronics. And the other side of that coin is that aluminum is a good choice for long overhead high voltage runs despite its lower conductivity, etc. Aluminum vs copper is a good example of different pros/cons in different scenarios. $\endgroup$ – Jason C Feb 28 '17 at 17:00
  • $\begingroup$ @JasonC The last sentence of the wiring paragraph already mentions Al being even cheaper than Cu and being used in some applications. I don't go into any more detail, though, as I'm worried that too many details might take this question about wood a little too far off the rails. $\endgroup$ – fenix d.Anconia Feb 28 '17 at 17:10
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    $\begingroup$ The ultra-futuristic genetic manipulation could also lead to a very interesting alternative of making it so that structures made out of the wood would keep the organism alive! Since the wood would be so hard it would be hard to harvest and process for usage, so instead the genetically enhanced trees are planted in the spot and grown into the desired shape, perhaps through a process that involves external tools such as lighting and lattices. $\endgroup$ – paolobueno Feb 28 '17 at 17:53
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Not to side step the "hardest wood" question, but as far as application... hardwoods are generally used in places where you want strength, but not weight. Things like tool handles, sports equipment (baseball bats, and hockey sticks for example), furniture, and so on.

A cubic foot of steel is incredibly strong, but it's also incredibly heavy at 490 lbs (7900 kg/m3). A cubic foot of hardwood is usually closer to 50 lbs (800 kg/m3).

As far as "the best material for the job" I've always had a preference for wood hockey sticks, but wood wrapped in Kevlar helps with wear and tear from... Well... Hockey. They're lightweight, slightly flexible, but still incredibly strong. My first stick is more than 20 years old and it's still usable despite some dings from rougher plays.

As far as construction goes... Of course steel and concrete are stronger and in many cases more durable, but they're much heavier and they're also much more expensive.

Also... The hardest wood possible really depends on the application and whether you're talking about a simple slab or a composite ply.

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  • $\begingroup$ Also: wood is seen as more environment friendly than concrete/steel, may resist better to extreme cold, ... $\endgroup$ – Matthieu M. Feb 28 '17 at 15:55
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Are you asking for the hardest wood or the strongest? They are two different things.

Bubinga is the strongest wood I know of, with a Modulus of Rupture (bending) of 24,410 lb$_\text{f}$/in$^2$ (168.3 MPa) but the crushing strength (compression) is only 10,990 lb$_\text{f}$/in$^2$ (75.8 MPa), less than half the bending strength.

Keep in mind that the crushing strength is very dependent on the orientation of the grain to the stress, the strength can be as much as ten times less when the stress is perpendicular to the grain.

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  • $\begingroup$ Would like to add two things here: 1) It may be possible use something like a hydraulic press to compress the would before hand, thus making it stronger. 2) It may be possible to make the wood stronger by monitoring/influencing it's growth. Eg. it would grow slower in cold weather - thus tighter year-rings and more compact wood. Also things like removing twigs and branches may effect it - this was previously done for example on trees intended to be used for window-frames. $\endgroup$ – Baard Kopperud Feb 27 '17 at 21:37
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    $\begingroup$ There is Ipe ("Brazilian Walnut" although not a true walnut) with Modulus of Ruputre 177 MPa, and Crushing Strength 93.8 MPa. $\endgroup$ – hatchet Feb 27 '17 at 23:44
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Lignum vitae has been used in engineering for centuries. As well as being dense and tough, it also has the unusual property of being self-lubricating.

Many hydro-electric turbines are still made using lignum vitae for bearings, and plenty of older hydro schemes are still in service with lignum vitae bearings after decades.

It was also widely used for lower-stress/lower-temperature bearings in cars and other vehicles. Track-rod/tie-rod ends in particular were always made of lignum vitae in pre-war cars, and this persisted well into the 1960s for some makes.

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    $\begingroup$ In passing: A couple decades ago I did a tour of the Point du Bois dam on the Winnipeg river. The dam had been built in 1911. The turbines were horizontal axis with the upstream end bearings made of Lignum vitae, and lubricated with a water drip. In 1983 they were still using the original bearings. $\endgroup$ – Sherwood Botsford Mar 1 '17 at 2:56
  • $\begingroup$ @SherwoodBotsford Interesting - thanks! $\endgroup$ – Graham Mar 1 '17 at 10:26
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If you just look for the hardest wood, you may want to look at petrified wood.

It would resemble wood it in appearance, but it would offer stone-like solidity. Think of using it for making pillars in a building, you would have a pillar which looks like a tree, but behaves like a stone.

Of couse you could hardly use it as a beam because of its poor resistance to traction.

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    $\begingroup$ Why? What does petrified wood offer the OP? As it stands, this is very little more than a link-only answer, as it simply reiterates one part of the question title and then gives a link to a Wikipedia article. $\endgroup$ – a CVn Feb 27 '17 at 12:31
  • $\begingroup$ @MichaelKjörling, thanks for your comment. I expanded the answer $\endgroup$ – L.Dutch Feb 27 '17 at 12:35
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    $\begingroup$ But petrified wood isn't wood any more, it's rock. By that criteria, wood-grained ceramic tile (used to be sold under the brand name of 'Strata') would be even harder. $\endgroup$ – jamesqf Feb 27 '17 at 21:09
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    $\begingroup$ Petrified wood doesn't have to look like wood or be brown. I own some examples that are the color of drywall. $\endgroup$ – person27 Feb 28 '17 at 4:44
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    $\begingroup$ How long does it take to petrify wood? Could you carve a wooden structure (beams, joints, etc) and then petrify the entire thing, then reassemble the stone pieces? Would that offer any advantage over carving stone in the first place? $\endgroup$ – Jason K Feb 28 '17 at 15:24
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In material science there is this picture of same force applied to different materials of the same dimensions.
The first answer to "how to make it to not break so easily is "double the amount of material or put a support where the force is applied".

So the real answer to your question is not where but why and how. For example the switch from wood ships to steel was dictated by the dimensions steel ship could have. On the other hand small ships were cheaper and lighter when made from fiberglass.
Same with houses, if you want to build quick, not sophisticated building you use prefabricates. But wood is more plastic and freely available. So you could grub your plot and have material on site already.

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  • $\begingroup$ You wrote "But wood is more plastic". I would edit to add the missing word after 'more', but I'm not sure what you were going to say there. $\endgroup$ – person27 Feb 28 '17 at 4:46
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    $\begingroup$ @Anon234_4521 he's trying to say that the wood is more plastic: plastic ˈplastɪk/ 2. (of substances or materials) easily shaped or moulded. (edited from en.oxforddictionaries.com/definition/plastic $\endgroup$ – MD-Tech Feb 28 '17 at 9:29
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Your questions are very broad, and have no definite answer. As mentioned in other answers, "hardness" doesn't have one single meaning.

The Wikipedia Hardness article mentions 3 main "types" of hardness, but even using one style of measurement machine, quite different (and conflicting) rankings will be observed. A hard material, for example, is pretty much useless if it softens in the rain or upon exposure to sunlight or just as it ages. Of course, you can protect a surface from sun, and rain (to some extent), but there are a fairly large number of properties that a substance must have to be "useful".

Also, and full disclosure here, I'm not a botanist, and have no knowledge of the Buloke, but Wikipedia says it's an Ironwood species. The same table that lists it at >5000 lists Ironwood at ~3000. You have to be very critical taking these numbers at face value. Ironwoods, I know a (very) little bit about. One of their properties is their high oil content. This is good for water (and bug) repellency but not good at all for painting or contact with other surfaces if they're prone to staining as (not if) the oil bleeds out.

As previous answer says, don't confuse hardness with strength. My guess for the "hardest" wood we could breed/engineer would be that its just as hard as the hardest biomaterial known. I think (but am not sure) this is either calcite, aragonite, or the stuff our teeth enamel is made of, hydroxylapatite. It would be interesting to determine if silica-based biomaterials were any harder, I wouldn't be surprised. (Diatoms and Radiolaria make silica walls). Since biomaterials are nanocomposites, and can be 10 times "harder" than the inorganic mineral they derive from, it's not really possible (imho) to say what the upper limit is for hardness.( Diatomaceous Earth is used as an abrasive, so it's probably pretty hard.)

For a material to be useful it not only needs a slew of properties to match a particular need, but the economics have to be favorable (meaning supply of the material good, and demand also strong).

The test you mentioned was (probably) designed (at least it was selected) to be useful with wood in the applications wood is used in. Meaning other measures would probably be required before a particular wood is deemed hard enough to function in some unusual, atypical way.

You ask two questions. The answer to the first is A. As far as what is now known, Wikipedia editors know more than I do, B. As far as what is possible, well, that's pretty much open-ended. Its certainly possible to make a plant develop a skin similar to the hard materials found in the animal (and microbiota) kingdoms. Find the hardest biomaterial known to man, and you can start there. If you want to speculate, increase its hardness by 10X.

To answer the second. You didn't give us all of its properties. As I said, giving us a single property and asking where it "might" be useful isn't likely to garner many incisive answers, its just far too broad and vague a question. As they say, the devil's in the details. Hard materials, generally are used to protect other materials from damage, or just the opposite, they are used to damage other materials. So, use as surface layers or in abrasives would be my first inclination.

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    $\begingroup$ I think that this answer would be significantly improved if you break it into paragraphs, use relevant formatting, and similar copy editing. Compare writers.stackexchange.com/q/26899/2533. $\endgroup$ – a CVn Feb 27 '17 at 12:33
  • $\begingroup$ I think I got the paragraph spacing correct but feel free to rollback the edit if you disagree. $\endgroup$ – EveryBitHelps Feb 27 '17 at 13:34
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I think I'm going to change "Hardest" to "Tough and Versatile" because we don't have a use case as yet to narrow things down. However, an extremely tough and versatile wood in North America is the Osage Orange Maclura pomifera. It's also known as the Hedge Tree. It is also the plague of anyone who has a need to cut one down in their own yard.

  1. The Hedge Tree is pretty tough, the toughest in North America with a Janka Hardness of 2040 when green and it gets harder as it dries out, reportedly up to 2700. This is about 2 times as much as the hardest of Oaks. I have seen it recommended that you do any carving when the wood is still green because you won't be able to when it dries.

  2. It is somewhat flexible. When combined with the hardness, it becomes a prized Bow makers wood. Native Americans would travel quite a long way to harvest limbs from an Osage Orange for bows.

  3. It is very resistant to rot. It is frequently used for fence posts because it will last below grade (in the ground) for a long, long time. It doesn't get mildew or molds getting deep into the wood. Bugs also seem to avoid the wood. The fruit is often used as a natural insect repellent.

  4. It's dense. This wood will eat chainsaws. I know this from having to cut one down in my yard. It was about 12 years old and it took 3 chainsaw chains to get through. Granted, I have a cheap chainsaw, but still. Incidently I still have a huge part of the trunk that is heavy as heck and I want to do something with it, but I don't know what yet.

  5. it burns hot! When used as firewood it will put out about twice the eat in BTU's as most varieties of oak. It pops a lot, so not good in an open fireplace, but in a sealed wood stove I was able to keep my house at about 80 degrees f during a snowstorm where outside was 12 f.

  6. It grows in a variety of climates and soils. It was used in the Midwest to create windbreaks and to help with soil erosion during the Dust Bowl.

So for what is possibly the toughest and most versatile natural wood that actually exists, Osage Orange Maclura pomifera is your friend. It would also make a good basis for any sort of monkeying around you may want to do with it's genes.

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As we're Worldbuilding, let's assume some development in genetic engineering, and the development of proteins to catalyse the assembly of carbon atoms in regular structures.

Then it's conceivable that our modified tree could build a stable tetrahedral crystalline form of carbon, at least on a cellular scale - perhaps as cell walls or an internal spine. As it's still a tree, these small structures would probably be embedded in a cellulose matrix, which may have its own weaknesses.

Nevertheless, their ultimate hardness would be that of their crystalline form - diamond.

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Southern live oak Quercus virginiana has a Janka hardness of 2,680 lbf (12,920 N) see http://www.wood-database.com/live-oak/ Not quite as 'strong' as some other species, but historically it was a very important component of American ship building because the long, curving limbs of the timbers could be made into ribs and other structural timbers without having to be carved. This gave great strength to the hull. Old Ironsides was an example of this kind of construction. Live oak was the secret weapon of American ship building. So part of the strength hardness question has to do with anticipated form.

Aircraft spruce has one of the highest strength:weight ratios of any natural material--somewhat different example, but also noteworthy.

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Supplementary answer. Wood is made of cellulose fibers. How strong is Cellulose?

Very strong. There's lots of detail here: https://www.extremetech.com/extreme/134910-nanocellulose-a-cheap-conductive-stronger-than-kevlar-wonder-material-made-from-wood-pulp . Quote: "lightweight, flexible, stronger than steel, stiffer than Kevlar" ... It is, of course, produced by a tree in a structure honed by evolution (long term) and environment (during the tree's lifetime) to be of best use to the tree. We have to do a bit of work to re-form it into nanocellulose, rather than just sawing it into beams.

Wood also contains a natural glue called lignin which bonds cellulose fibers together. Just as cellulose is at least as strong as our best plastics, lignin is at least as good as our best glues and resins. Until recently, it was an order of magnitude better, but our chemists have caught up, and now we can glue wood to wood as strongly as if the tree had grown the wood into the shape we wanted(*). Enter Glulam. (Horrid name: a contraction of glued laminate, I think). Anyway, google "glulam" and you'll find that people are now building small skyscrapers out of wood, and planning larger ones. It is, after all, weight for weight as strong as steel (and counter-intuitively, more fire-resistant! ) Glulam is not the same thing as simple sawed timber, so architects are still feeling their way, and building experience and confidence with smaller structures first.

This is world-building, so these references tell you what is possible (using cellulose). We may be able to re-program trees to grow wood more suited to our own needs, using genetic engineering. Or on a planet with higher gravity, evolution may have done the same (otherwise, there are no trees on that planet). And it's even possible that there's a better bio-polymer out there than cellulose.

(*) by the way, mediaeval builders used the shapes that the tree grew. They didn't hack the wood into arbitrarily straight but weaker timbers. They built arch-like roofs and ships containing naturally curved timber. Occasionally, they'd even tinker a bit with the shape that the tree was growing into while it was small and pliable, then wait a century to harvest timber with the curves they needed. We may now (or soon) have the biotechnology to steer the growth of a tree by more subtle means than tying a sapling to a framework. However, do we have the patience?

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So not sure about which wood is hardest but here in Australia the ironwood species have historically been used for power poles. Their density and strength means that they are highly resistant to rotting (less of an issue in the outback) and termite attack (more importantly). Using steel is not that good an option as it conducts electricity. Decades ago the ironwoods were more accessible as you could cut them from the surrounding environment. Of course they have been over harvested and being a very slow growing tree are no longer a sustainable wood product. Also we're better at insulating steel poles and forming them out of cast concrete.

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