Diamond is not suitable for applications that involve very high temperatures. It is metastable and thus starts to turn into graphite.
Other diamondoid materials like the carbides of the titanium vanadium and chromium group (interstitial carbides) can be used for high temperature applications. Materials that retain their structural strength at high temperatures are called refractory (wikipedia).
About consistent design
Obviously a systems should be designed such that there are no single parts that limit the temperature resilience way below the potential. This is a special case of "consistent design for external limiting factors". Complete sets of high temperature DMEs are needed) - single ones have no use.
That a material does not melt does not mean that it shows no surface diffusion.
Stability of free or mutual contacting or environmentally contacting passivated surfaces (that are possibly strained) will reduce the allowed temperatures well below the bulk material melting points. Interstitial diffusion may too be a limiting factor.
For really high temperature applications minimal sized DMEs will thus likely not work. Bigger scale (interlocking) refractory tiles will still be usable though. But they'll need regular replacement before they fuse together. Disposal of them might proove diffecult.
List of some refractory materials
- SiC Moissanite – Hardness at 20°C: Mohs 9.25 – Melting point: 2730 °C (decomposes)
- ... boron carbides ...
- TiC (3,160 °C; 5,720 °F; 3,430 K; abundant elements, simple cubic) (passivation layer formation may be an issue. see further down - external links)
- VC (2810 °C; 9-9.5 Mohs, cubic)
- Cr3C2; Cr7C3; Cr23C6 (1,895 °C; 3,443 °F; 2,168 K; extremely hard; very corrosion resistant)
- ZrC (3532 °C; extremely hard; highly corrosion resistant; very metallic, cubic)
- Nb2C (3490 °C; extremely hard; highly corrosion resistant)
- Mo2C (2692 °C) ; MoC; Mo3C2 
- HfC (3900 °C; very refractory; low oxidation resistance, cubic)
- TaCX (3880 °C (TaC) 3327 °C (TaC0.5); extremely hard; metallic conductivity, cubic) – tantal is very rare
- WC (2,870 °C; 5,200 °F; 3,140 K; ~9 on Mohs scale, hexagonal)
- Ta4HfC5 (record holder: 4,215 °C; 7,619 °F; 4,488 K)
Note: Many elements here are neither abundant nor prime targets for mechanosynthesis.
- add notes on SiC
- add notes on recycling and disassembly
- add notes on self repair
- Maybe in ovens for recycling of diamonoid waste that ended up in a state beyond repair.
Usage in extreme environments
- For robots operating on the surface of Venus. e.g. for mining.
Spacecraft flying close to the Sun (speculative)
Our sun "sol" has a surface temperature of 5505°C or 5778K. Seen relatively this is only a bit above the melting point of the highest melting materials known. By adding:
- strong electromagnetic shielding against the solar wind (poles?)
- highly reflective mirror
- active heat pump for cooling through magnetic nozzles with temperatures >>5000K
It seems not entirely implausible for a space-probe to slightly dip into the atmosphere of the sun or even pernanently stay in a low solar orbit (LSO) just high enough to not loose too much speed by atmospheric drag. (Protuberances might be problematic)
Probes for researching the deep liquid interior of planets (very speculative)
It is believed that the outer core is at temperatures from 3000K to 5000K so there are regions where some refractory materials would not melt where they under ambient pressure. But they are not! (TODO: find highest melting point materials at high pressures)
Active cooling is necessary for cooling the inner workings of the probe. It makes the problem of high temperature even worse since the cooling exhaust gets even hotter than the environment and unlike in space there can't be any magnetic shielding against heat levels that no physical material can take. Pumping "cooling magma" (what a word) quicker might only help till the point the friction through viscosity itself causes too much heat. Actually its very likely that the environment is so viscous or even quasi solid that pumping the embedding medium is completely impossible.
To withstand the immense pressures such a probe would need to be devoid of any macroscopic cavities. Sparse nanoscale voids necessary for any atomically precise nanoscale activity should be ok. Materials that seem incompressible in our everyday environment might collapse into other crystal structures. This has to be taken into account. Water ice is a good and well researched example. Quartz too. Cristobalite a high pressure modification of quartz (that is safely metastable at ambient pressure) may be a good building material for the inner cooled structures since it wont't collapse any further under high pressure. Slowly increasing pressure and temperature can lead to thermodynamic equilibration processes in crystal structures so at least for the outer non-cooled hot refractory hull there must be used something that is not too vulnerable for such degradation.
A great challenge is that highly efficient thermal isolation often depends on lots of voids in a material wich are problematic at extremely high pressures.
As energy source only fission seems to be the only possible and also a good option. Fission the only possible option because fusion power would need large voids, geothermal cannot work for cooling and magma current gradients (the analogue to wind down there) are to small on the size-scale of the probe. The probe must be kept compact with low surface to volume ratio. Thus the bigger the probe is the better. Stretching it long for flow gradient energy harvesting wold cost unproportially more in cooling its interior. Fission is a good option since the depths of the planet should be rich on heavy fissile and fertile elements.
The outer refractory surface will loose its outermost layer continually. Especially worrisome is that the medium of the environment could lower the melting point of the hull and dissolve it (e.g. iron likes to dissolve carbon)
For the probe to be feasible it must be capable to continually rebuild its hull at sufficient pace and push it from the inside out with only a few sparse nanoscapic voids to work in. Also it must be capable of sucking in magma and filtering out of that chaotic mix of elements the useful ones for the refractive hull and nuclear fission power. Moving from magma into machine phase seems highly nontrivial.
- Many interstitial carbides are refractory materials. See wikipedia: Carbide.
The may be susceptible to creation of passivation layers. Titanium Carbide can form a titanium dioxide layer. See paper: Reaction of titanium carbide with water.
(TODO: find out whether this happens only at fault locations and at which temperatures this starts to occur)
- Ultra-high-temperature ceramics