Kyanite in printed silica cores – expansion on decomposition offsets shrinkage

Silica cores make up more than 90% of all cores for superalloy turbine blades. They have low thermal expansion and good thermal shock resistance, sinter well, keep their dimensions at temperature and leach easily in caustic. Vat photopolymerisation (VPP, the stereolithography family) makes complex shapes possible without core boxes – one of the key technologies for hollow blades with thin cooling passages.

Printed silica cores have two well-known problems, though. Heated above 1500 °C they shrink noticeably: around 1200 °C densification by viscous flow of the glass begins, causing shrinkage and creep. Around 1350 °C the crystallisation of amorphous silica into cristobalite speeds up, strengthening the core – but that crystallisation alone is not enough for single-crystal casting. Silicon carbide, silica and mullite fibres, iron oxide and alumina have all been added to speed up cristobalite formation and raise creep resistance.

Authors at Tianjin University, with the Beijing Institute of Aeronautical Materials (BIAM), propose another route – adding a mineral that expands on heating and so offsets the shrinkage. That mineral is kyanite.

How the study was run

The printing feedstock was fused silica, kyanite (with mullite for comparison) and zircon in an acrylate photosensitive resin. Zircon was 5% in every mix, and kyanite or mullite replaced silica from 0 to 20 wt% (samples C0, K05–K20 and M05–M20), sintering temperatures from 1150 to 1250 °C. Printing was followed by debinding and sintering.

Everything that matters for a core was tested. Strength by three-point bending over a 30 mm span at room temperature and at 1540 °C. Sag on 120 × 6 × 2 mm bars over a 100 mm span after heating to 1540 °C at 10 °C/min and holding for 1 h, to the Chinese aviation standard HB 5353.4-2004. Thermal expansion in stages from room temperature to 1580 °C and back. Porosity and leachability after a simulated casting heat treatment, to HB 5353.6-2004. Structure by SEM, phases by quantitative XRD with an internal standard.

Best variant: 15% kyanite, sintered at 1225 °C

3.06%sintering shrinkage

0.82 mmsag at 1540 °C after 1 hour

27.05 MPaflexural strength at 1540 °C

78.85composite quality index CQI, the best in the study

Structure and phases

Kyanite and mullite hinder densification. At 1150 °C all samples are full of pores. At 1250 °C the core without additions sinters much denser, while the one with 15% kyanite stays looser. Kyanite is evenly distributed through the volume – the authors checked that separately.

Cristobalite matters most. Without additions there is almost none at 1225 °C. With kyanite its fraction rises from 0 to 24.42% as kyanite goes from 0 to 20%; with mullite, to 29.22%. At 15% addition it is 21.37% with kyanite and 27.72% with mullite. The difference comes from the alumina content: 63.1% in kyanite, 71.8% in mullite. Both minerals dissolve in amorphous silica particles and speed up their crystallisation into cristobalite, and the more Al₂O₃ in the additive, the stronger the effect. At 1250 °C the cristobalite content of kyanite and mullite samples converges: kyanite decomposes at that temperature and releases silica that quickly turns into cristobalite.

Expansion, shrinkage and sag

The more kyanite or mullite, the smaller the overall linear shrinkage on heating. The authors split the expansion curve into seven stages and computed the dimensional change in each. In the first stage, from room temperature to 220 °C, the additive-free sample sintered at 1150 °C barely changes (–0.01%), while K15 sintered at 1250 °C expands by 0.343% and M15 by 0.381%: the more cristobalite, the stronger its abrupt phase change on heating.

The main effect comes above 1300 °C. Kyanite decomposes with a volume expansion, and above 15% that expansion markedly offsets sintering shrinkage between 1300 and 1400 °C. The columnar mullite crystals formed by decomposition interlock into a framework – which, according to the authors, is the main source of high-temperature strength and sag resistance.

The composite score

Core requirements pull against each other: less shrinkage means more porosity, more high-temperature strength means poorer leaching. To compare variants, the authors combined eight indicators into a ceramic core quality index (CQI): sintering and "casting" shrinkage, thermal expansion, sag, porosity, leachability and strength at two temperatures. Each indicator is normalised – some "lower is better", others "higher is better" – and weighted; sintering shrinkage carries the largest weight, 0.25.

The top score, 78.85, went to 15% kyanite sintered at 1225 °C: sintering shrinkage 3.06%, "casting" shrinkage 0.86%, sag 0.82 mm, flexural strength 10.06 MPa at room temperature and 27.05 MPa at 1540 °C.

There is a flip side. Sintered at 1250 °C, shrinkage drops to 2.94%, but porosity rises to 28.83% – a trade-off the authors call unavoidable. Too much kyanite hurts strength through microcracks and excess porosity.

Leachability first rises with kyanite content and peaks at 15%: kyanite decomposition increases open porosity after casting, giving the caustic capillary paths into the core. Beyond that it falls – porosity stops rising, while excess kyanite turns into mullite, which is practically insoluble in caustic.

Limits

This is a laboratory study on test bars. CQI is the authors' own aggregate, and its weights decide which variant is called "best". The authors state that the process fully meets the requirements of aero-engine casting, but the paper contains no blade pours: casting was simulated by heat treatment. Nor does it show a real core of complex shape – with thin airfoils and ties, where sag and leaching behave differently from a bar.