Why Nitride Bonded Silicon Carbide Lose Strength After 8–12 Months in Oxidizing Atmospheres

I’ve spent twelve years dealing with high-temperature equipment around Shanghai and the surrounding ceramics and metallurgy plants. One of the most consistent problems I see with nitride bonded silicon carbide is that the kiln shelves, plates, and setters look solid when they go in, yet after several months of normal service they start cracking or sagging under loads they previously handled without issue. Most operators assume the material simply “wore out” or that the firing cycle was too aggressive. In the lines I work with, the real driver is progressive oxidation of the nitride bond phase once the surface is exposed to air at temperature.

Nitride bonded silicon carbide relies on the silicon nitride matrix to hold the silicon carbide grains together and give the part its strength and thermal shock resistance. In a reducing or neutral atmosphere that bond stays relatively stable. Once the kiln or furnace runs with excess air—common in intermittent ceramic firings or certain metal heat-treatment cycles—oxygen attacks the nitride. A thin silica-rich layer forms on the surface and, more importantly, at grain boundaries. The part does not suddenly fail; it gradually loses flexural strength and becomes more sensitive to thermal gradients. I regularly see plates that measured well above 40 MPa when new drop into the low 20s after 8–12 months of daily cycling between 1200 °C and room temperature.

The practical consequence shows up in two ways. First, the shelves develop hairline cracks that start at the edges or around support points and then propagate under their own weight plus the ware load. Second, the effective load-bearing capacity falls even before visible cracks appear, so operators either reduce the number of pieces per shelf or accept higher breakage rates. In both cases the plant ends up replacing furniture sooner than the original purchase calculation assumed. Plants running continuous pushers or roller kilns with tighter atmosphere control usually get longer life from the same grade; the intermittent batch kilns common in the Shanghai region show the problem faster because every cool-down lets fresh air reach the hot surfaces.

What actually slows the degradation is straightforward once you treat the bond phase as the limiting factor. Keep peak temperature and time-at-temperature as low as the process allows; every extra 50 °C above 1350 °C noticeably accelerates the oxidation rate I measure on returned parts. Where the kiln design permits, a short purge or a slightly reducing atmosphere during the high-temperature hold reduces the oxygen partial pressure enough to extend usable life by several months. Equally important is mechanical loading. Nitride bonded sic that are supported only at the ends and carry heavy, uneven ware loads develop higher tensile stresses on the bottom face; once the surface has been weakened by oxidation those stresses initiate cracks earlier. Moving to three-point or continuous support, or simply reducing the maximum load per plate by 15–20 %, often restores the original replacement interval without changing the material grade.

I also check returned plates for the thickness of the oxidized layer. A uniform light-gray surface film of a few hundred microns is normal; localized thicker scale or powdering at edges usually means either a hot spot in the kiln or prolonged exposure above the recommended limit. Plants that log atmosphere readings and maximum shelf load alongside the number of cycles before first cracks appear can usually push average life from roughly 10 months to 14–16 months on the same nitride bonded sic. The ones that keep running the original dense loading pattern and full air atmosphere simply replace plates on a shorter, more expensive schedule and keep wondering why the new set lasts no longer than the last one.

The material itself is capable of long service when the bond phase is protected. The difference I see in the field is almost never a sudden change in raw material quality; it is whether the plant treats oxidation and loading as controllable variables or as inevitable background conditions.