Michael J. Pomeroy

Formation of LnSiAlON glasses and their properties

Abstract The preparation details of bulk glasses in the Ln-Si-Al-O-N systems (Ln = Y, Ce, Nd, Sm, Eu, Dy, Ho and Er) containing 17 equivalent % nitrogen are reported. The properties of these oxynitride glasses are examined in detail. Changes observed in density, molar volume, hardness, thermal expansion and glass transition temperature are presented and are found to vary linearly with the cationic field strength or ionic radius of the rare earth modifier. The implications for changes in glass structure caused by the substitution of one rare earth cation for another are discussed in relation to the property changes observed.

Microstructural development and surface characterization of electrodeposited nickel/yttria composite coatings

The addition of yttria particles to a Watts nickel plating bath is shown to significantly change the surface morphology and preferred crystallographic orientation of the composite coatings deposited on Inconel 625 substrates. The typical morphology for nickel deposits from an additive-free Watts bath is pyramidal, however the addition of yttria particles in concentrations >2 g dm -3 changes this morphology to hemispherical. The preferred growth direction was also influenced by yttria with a change from a to a preferred growth direction occurring when the particle loading exceeded 2 g dm -3 . The results here show that both the nucleation and growth process of the nickel matrix was greatly influenced by the yttria additions. When yttria was absent from the electroplating bath the growth took place preferentially on (200) orientated planes. However, yttria acts to inhibit growth on grains of this orientation, leading to a requirement for additional nucleation, which takes place preferentially on (111) planes. This constant inhibition of growth and renucleation causes the changes to a hemispherical morphology observed.

Effect of composition on the properties of glasses in the K2O–BaO–MgO–SiO2–Al2O3–B2O3–MgF2 system

Abstract Twenty five glasses in the(1−Z)BaO:ZK2O:(6−X)MgO:XMgF2:(3−Q)Al2O3:QB2O3:8SiO2 system have been prepared and the effect of systematic changes in composition (Z=0, 0.25, 0.5, 0.75 and 1.0, X=2, 2.5 and 3.0 and Q=0, 0.5 and 1) on molar volume (MV), fractional glass compactness (C), microhardness (μHV), glass transition temperature ( T g mid ) and coefficient of thermal expansion (α) determined. As potassium is substituted for barium (increasing Z), increases in MV and α, and decreases in C, μHV and T g mid arise which are attributed to the replacement of ONB–Ba–ONB ionic bridges by two ONB–K terminations (ONB = non-bridging oxygen). When fluorine is substituted for oxygen, reductions in μHv and T g mid and increases in MV and α values occur. These property changes occur because of reduced crosslink densities associated with the replacement of ≡Al–O–Si≡ crosslinks by ≡Al–F terminations. In general, the substitution of aluminium by boron results in decreases in MV, μHV and T g mid values and increases in α values. These effects are attributed to boron assuming a tri-coordinated state in the glass, giving rise to reduced crosslink densities, together with the release of modifier cations (Mg, K and Ba) from their charge balancing role for four-coordinated aluminiums leaving them free to cause greater network disruption.

Joining of ceramics using oxide and oxynitride glasses in the Y-Sialon system

Glasses in the Y-SiAlO and Y-SiAlON systems, of standard cation composition (28:Y, 56:Si, 16:Al) have been investigated as potential adhesive materials for silicon nitride-based ceramics. Glass transition and crystallisation temperatures are reported. Hardness and density have also been measured. These glass materials have been used to bond hot-pressed and sintered β-sialon ceramics containing an intergranular glass composition similar to that of the adhesive material. Joints were formed via two processes: (1) hot-pressing at temperatures from 1130°C to 1600°C for 1 h at a pressure of 5 MPa using the Y-sialon glass; and (2) pressureless sintering at 1600°C for I using both the Y-sialon and Y-sialo glasses. At a temperature of1600°C, SEM analysis illustrates that both processes yield visibly good joints. Hardness measurements indicate that there is no degradation in value between the parent ceramic and the bonded region. Joining was dominated by the flow of glass across the ceramic interfaces, dissolution of the individual grains within the glass and reprecipitation of the Si 3 N 4 across the interface reinforcing the joint. 1997 Elsevier Science Limited.

Kinetics of weight changes and morphological developments during oxidation of pressureless sintered β-sialons

Abstract The oxidation behaviour of yttria densified β-sialon materials (Si(6 − z)AlzOzN(8 − z) with z-values of 0.2, 0.5, 1.0, 1.5, 2.0 and 3.0 at 1350 °C is reported for times up to 256 h. The sialons can be grouped into two categories with respect to oxidation kinetics and predominant crystalline phases formed in the surface layers. For the z = 0.2 to 1.0 materials, changes in oxidation parabolic rate constant occur after 128 h and yttrium disilicate or cristobalite are the major crystalline oxidation products within the surface layers. For the higher z-value materials, no changes in parabolic oxidation rate are observed and mullite is the predominant phase formed. Examination of cross-sectioned specimens of the z = 0.2 and 3.0 materials indicates changes in morphology consistent with those reported for the surface layers. The kinetics and mechanism of oxidation and morphological development of the oxide scale are discussed in terms of additive diffusion effects.

Potential of Nd-Si-Al-O-N glasses for crystallisation to glass-ceramics

Abstract U-phase Ln3Si3 − xAl3xO12xN2−x (Ln = La, Ce, Nd) occurs as a major devitrification product in rare earth sialon systems. Heat treatment studies have been carried out at temperatures corresponding to optimum nucleation and crystal growth on Nd-U-phase compositions. Two starting glass compositions for U-phase with varying cation ratios, Nd:Si:Al = 1:1:2 and Nd:Si:Al = 1:1:1, containing 20 eq.% N were prepared and their densities, hardnesses and glass transition temperatures measured. Detailed nucleation and crystallisation measurements were carried out using a two-stage heat treatment process employing differential scanning calorimetry and a tube furnace. Optimum nucleation temperatures and the activation energies for crystal growth were determined and the influence of sample-specific surface on the devitrification mechanisms assessed. The composition with 1:1:2 atomic ratio was further investigated to assess the effect of varying nitrogen contents in the mixtures. This investigation revealed the existence of a small range of unit cell dimensions for U-phase with variation in nitrogen content. The refractoriness and microstructure of the resulting phases are described and their potential as grain boundary phases in silicon nitride based ceramics discussed.

Effect of gaseous environment on the corrosion of β-sialon materials

Abstract This paper reports the effects of different gaseous environments on the corrosion behaviour of β-sialon materials. Six β-sialon materials with z-values of 0.2, 0.5, 1.0, 1.5, 2.0 and 3.0 were prepared by pressureless sintering. Corrosion results obtained at 1350 °C in environments comprising (i) Laboratory air; (ii) Argon — 20% oxygen — 2% chlorine; (iii) hydrogen — 10% hydrogen sulphide — 2% water vapour and (iv) nitrogen — 15% carbon dioxide — 4% oxygen — 0.2% sulphur dioxide showed that for each of the four environments the z = 3.0 material exhibited the best corrosion resistance. The mechanism of corrosion at 1350 °C in each of the four environments involves the formation of large volumes of low viscosity liquid phases. These large volumes of liquid enable the solution of β-sialon grains to occur and it is this process which is responsible for corrosion.

An Introduction to the Glass Formation and Properties of Ca-Si-Al-O-N-F Glasses

Ca-Sialon glasses have been known for some time [1] and they are effectively calciumalumino- silicate glasses containing nitrogen which improves their mechanical properties. Calciumalumino- silicate glasses containing fluorine are known to have useful characteristics as potential bioactive materials [2]. Therefore, the combination of both nitrogen and fluorine additions to these glasses may give useful bioglasses with enhanced mechanical stability.Addition of fluorine to oxynitride glasses was not reported previously and this paper gives the first report of the glass forming regions (and evaluation of some properties) in the Ca-Si-Al-O-N-F system. Within the previously defined [1] glass forming region in the Ca-Si-Al-O-N system, homogeneous, dense glasses are formed. Addition of fluorine extends the glass forming region but also increases the reactivity of the glass melts. One major problem is fluorine loss as SiF4, but also loss of nitrogen, which affects the final composition and results in porous samples. To suppress the fluorine loss and CaF2 precipitation, consideration of the ratio of cations to fluorine and the coordination number of Al atoms is important. Discussion of the role of cations in these oxyfluoronitride glasses is presented.

Microstructural Transformation in Platinum Aluminide Coated on CMSX-4 Superalloy

Microstructural evolution induced by isothermal exposure at 1100°C in a platinum aluminide coating was investigated. Specimens of CMSX-4 alloy have been platinum aluminised to give a single phase (Ni,Pt)Al coating. Coated alloys were exposed in an oxidising environment for 188, 350, 750 and 1500 hours at a temperature of 1100oC. After isothermal oxidation of the single phase (Ni,Pt)Al coatings the surface is shown to progressively roughen. The compositional development assisted by chemical diffusion during isothermal oxidation at 1100°C has been related to the transformation of the coating from its original β-NiAl (B2) structure to a Ni-rich (L10) martensite γ′. The transformation took place due to aluminium depletion in the coating from the formation of the Al2O3 scale and interdiffusion between the coating and alloy substrate. The effects of phase changes on coating surface character are considered. Introduction The development of nickel–based alloys for use in gas turbines with increased gas entry temperatures and therefore improved efficiencies has been realised by the continuous optimisation of alloy compositions as well as innovative processing techniques, such as directional solidification or single crystal solidification routes [1-3]. Whilst optimised for creep resistance and high strength, alloys used in gas turbines have negligible intrinsic corrosion or oxidation resistance and therefore require protection by coatings, which are typically based on β-NiAl [1]. Platinum modified β-NiAl coatings dramatically improve the hot corrosion and oxidation resistance of the aluminide coatings [4-8] and there has been much development of Pt modified aluminide coatings since their original development and commercialisation in the early 1960s and 1970s [9, 10]. During exposure at high temperatures in an oxidising environment, the protective coatings become aluminium depleted due to aluminium consumption by the formation of an α-alumina protective scale and due to either Al diffusion into the substrate alloy or nickel diffusion into the coating which results in the formation of Ni3Al (γ’) regions within the coating [11,12]. The transformation of the aluminide coating to the less protective γ’ phase results in the coating losing its protective character since the aluminium levels become too low to sustain the exclusive development of an α-alumina protective scale. Pt-modified aluminide coatings subjected to oxidation frequently exhibit surface roughening, which is referred to as rumpling. Original explanations for this phenomenon were based upon repeated oxide spallation [13] or thermal expansion mismatch between the coating and alloy substrate [14]. More recently, rumpling has been associated with volume differences associated with phase changes occurring in the coating. Thus, Tolpygo & Clarke [12] attributed rumpling to the β to γ’ transformation giving rise to volume changes, associated stresses and thus deformation of the coating surface whilst Zhang et al. [15] and Chen et al. [16] attribute rumpling to a martensitic transformation arising in β-NiAl which occurs when a specific aluminium content is reached. Such Materials Science Forum Online: 2004-08-15 ISSN: 1662-9752, Vols. 461-464, pp 343-350 doi:10.4028/www.scientific.net/MSF.461-464.343

Oxidation studies of a high Al substituted β-sialon

Abstract The oxidation behaviour of a z=3 β-sialon material in air has been studied at a temperature of 1350°C for periods of 1, 8, 16, 32, 64, 128 and 256h. The oxidation kinetics exhibit a straight line relationship between the square of the specific weight gain and time and the parabolic rate constant evaluated from the slope of this line was found to be 4.277 × 10 −12 g 2 cm −4 sec −1 . The morphology of the exterior surface layers indicates oxynitride liquid formation during the initial stages of oxidation. The phase assemblage of the surface products further indicates the partial oxidation of sialon to silica, mullite, X-phase and Y 2 SiAlO 5 N. As the time of oxidation increases, needle like mullite crystals embedded in a yttrium rich glass phase becomes the predominant phase after 16 hours. In cross section the exterior oxide scale comprised mullite and silica crystals in a yttrium containing glass. Beneath this layer silica levels are increased and mullite levels appear to decrease. The formation of the yttrium rich amorphous phase is accompanied by the occurrence of an yttrium depleted layer at the ceramic-oxide interface. Cooling rates appear to have little influence on the phase assemblage since no major difference in the phase assemblage was observed for furnace cooled and rapid (air) cooled specimens after 1 hour. The kinetics and mechanism of oxidation and morphological development of the oxide scale are discussed in terms of additive diffusion effects and phase relationships exisiting in the YSiAlON system.