Hard Porcelain Research Articles

Alkali-ions diffusion, mullite formation, and crystals dissolution during sintering of porcelain bodies: Microstructural approach

The effect of alkali-silicate glassy matrix as replacement for feldspar in soft and hard porcelain compositions was studied. SEM and X-ray diffraction analysis were used to evidence phase evolution. For each composition, the influence of soaking time was evaluated. The difference in chemical composition (amount of alkali and alumina) between the two types of porcelain studied influenced the final microstructure: density, pore size and shape, and mullite content. Quartz dissolution was more important in soft porcelain where the mullitization was limited by the low amount of alumina compared to hard porcelain. Replacing the feldspar by alkali-silicate glassy matrices with similar chemical composition, the amount of secondary mullite and mechanical properties increased in both soft and hard compositions.

Wet or dry mechanochemical synthesis of calcium phosphates? Influence of the water content on DCPD–CaO reaction kinetics

Mechanosynthesis of calcium phosphates can be performed under wet or dry conditions. In most papers and patents, grinding under wet conditions was selected. So far, only a few papers were devoted to dry mechanosynthesis of calcium phosphates. To understand why wet mechanosynthesis was preferred, the influence of water addition on the kinetics of the mechanochemical reaction of dicalcium phosphate dihydrate with calcium oxide was investigated. The DCPD disappearance rate constant k and the final reaction time t f were determined in each case and correlated with the water content present in the slurry. Results showed that the addition water (i) slowed down the reaction rate and (ii) increased the powder contamination by mill material (hard porcelain) due to ball and vial erosion; and that (iii) wet milling did not generate the expected products, in contrast to dry grinding, because porcelain induced hydroxyapatite decomposition with the formation of β-tricalcium phosphate and silicon-stabilized tricalcium phosphate. Consequently, dry mechanosynthesis appears preferable to wet milling in the preparation of calcium phosphates of biological interest.

Non-contact dilatometry of hard and soft porcelain compositions

Two different porcelain compositions were studied: a soft and a hard one. DTA, optical non-contact dilatometry and DSC were used to evaluate the thermal behaviour of the porcelain compositions with the aim to master the suitable thermal cycle for each. Results were interpreted on the basis of mineralogy and microstructure: the amorphous phase, abundant in soft porcelain, plays an important role on thermal expansion (8·10–6 K–1 for soft and 6·10–6 K–1 for hard at 1000°C). Thermal expansion behaviour as function of firing time was also studied. Non-contact dilatometry characterisation of porcelain bodies enable to master the suitable thermal cycle for the sintering.

The influence of incorporation of ZnO-containing glazes on the properties of hard porcelains

New ZnO-containing glazes were developed, characterized and incorporated (5–8 wt.%) in conventional hard porcelain formulation for producing tableware porcelains. The experimental results showed that glaze additives enhancement the densification of porcelains. Maturing temperature range was widened due to the dissolution of quartz crystals. The modified porcelains featured better mechanical properties and whiteness and lower shrinkage than conventional hard porcelains. Good matching of thermal properties between porcelain body and glaze resulted in successful glazing.

Characterisation of porcelain compositions using two china clays from Cameroon

Mayouom and Ntamuka china clays, from Cameroon were used to produce porcelain bodies. Two soft porcelain formulations: PSI (with Mayouom) and PSII (with Ntamuka) and one hard porcelain, PH (with both Mayouom and Ntamuka) were prepared. The maximum density and flexural resistance for these formulations were obtained at 1200 °C for PSI, 1225 °C for PSII and 1350 °C for PH. Their properties and values were, respectively, density (2.42, 2.58 and 2.59 g/cm 3), water absorption (0.15, 0.15 and 0.02%), porosity (4.3, 5.3 and 4.2%) and flexural strength (148, 148 and 160 MPa). In addition to varying amounts of liquid phase, the soft porcelain formulations contained mullite and quartz crystals while the hard porcelains contained quartz and more intense mullite peaks. At 1250 °C PSI and PSII presented a self-glazing phenomenon which gave significant brightness and high aesthetic quality. PH with lower alkali (especially Na +) did not self-glaze even at 1400 °C. Considering the soft porcelain formulations, the higher amounts of TiO 2 (0.83%) and Fe 2O 3 (0.31%) in PSI (with Mayoum clay) resulted in a higher sintering effect at lower temperatures, giving lower water absorption and higher resistance. Both Mayouom and Ntamuka china clays were found to be suitable raw materials for the production of porcelain stoneware tiles. Ntamuka could also be used for the manufacture of optimal quality whitewares.

The effect of fired scrap addition on the sintering behaviour of hard porcelain

The effect of the addition of porcelain fired scraps on the sintering, phase composition and properties of a hard porcelain was studied. Two ceramics were compared: a commercial triaxial porcelain labelled C-0 and one, where 15% of the mass of C-0 was substituted by scraps yielding a composition labelled C-15. The sintering was evaluated by the linear shrinkage, the water absorption and the variation of closed and total porosity, while the phase transformations were investigated by DTA-TG, dilatometry and XRD. Samples, sintered at 1350 and 1400 °C, were used for the evaluation of the Young modulus and the bending strength. The addition of the fired scrap had a positive effect on the quartz dissolution and on the formation of the liquid phase during the sintering. The sintering and the mechanical properties were improved by heat treatment in the 1300–1350 °C range. Above this temperature, a significant over-firing with a decrease of the mechanical properties was observed.

Design and development of a chamotte for use in a low-clay translucent whiteware

In the design of a translucent whiteware it was intended to combine the best features of bone china and hard porcelain, i.e. to be aesthetically pleasing and to have the resistance to edge chipping of the former with the scratch resistance of the type of glaze used on the latter. The design envisaged producing a whiteware from two chamottes, each having the same composition but with different median particle sizes, and only a small amount of clay (∼10 to 15%) to minimise the firing shrinkage and its anisotropy. In the design of the chamotte composition, anorthite (CaO·Al 2O 3·2SiO 2) was selected to be the major phase and mullite (3Al 2O 3·2SiO 2) and glass the minor phases. A further design requirement was to have high crystalline (⩾70%) and low glass (⩽30%) contents to enhance the fracture toughness of the whiteware. Anorthite was chosen as the major phase because its refractive index would be similar to that of the glass phase, which enhances translucency, and its thermal expansion coefficient (TEC) allows the TEC of the whiteware to match that of some glazes made with high-silica contents. Mullite was found to reduce sagging during glost firing.

Optical characteristics of Japanese porcelain bodies prepared from high plastic clay

Abstract The absorption and scattering coefficients of porcelain bodies were measured with the aim of improving the optical characteristics of Japanese porcelain prepared from plastic clay. The spectral absorption of the bodies was 0·02-0·07 mm-1, similar to that of European hard porcelain. It was found that absorption could be lowered by increasing the content of alkali metal. Scattering coefficient, 10 mm-1 at 380 nm and 3·0 mm-1 at 780 nm, was much smaller than that of European bodies, which is typical for porcelain produced from Japanese plastic clay. Scattering coefficient varied with wavelength, behaviour which was considered to result mainly from the amount of mullite crystals present. On a reflectance-transmittance diagram, all the data for the Japanese porcelain bodies were on a single curved line, again demonstrating typical optical characteristics for Japanese porcelain bodies produced from plastic clay. Based on these results, a way of improving the whiteness and translucence of Japanese porce...

Rheological study of waste porcelain feedstocks for injection moulding

Five types of hard porcelain powders for injection moulding have been studied, using the same commercial binder in all cases. These powders contained different amounts of recycled porcelain, since one of the principal aims of this work is to reuse waste porcelain in the injection moulding process. The evaluation of the flowability of different feedstocks was carried out using rheological parameters, like critical powder volume concentration (CPVC), melt viscosity, activation energy, yield stress, powder law index and rheological index. The correlation of these parameters with injection behaviour was established, in order to confirm the possibility of using this moulding process with recycled porcelain powders.

Fracture toughness of bone china and hard porcelain

The fracture toughness of unglazed, commercial samples of bone china and hard porcelain produced by powder pressing spray dried granulated bodies has been determined using the Vickers indentation m...

Effect of feldspar concentrate composition on structure and properties of ceramic mixtures

Feldspar concentrates produced from concentrated rocks of Karelia are investigated as components of hard porcelain mixtures. The effect of feldspar concentrates having different ratios of potassium and sodium oxides, the mineralogical composition, and the structural specifics on the structure and properties of porcelain mixtures is studied. It is established that the grain structure and the ratio of quartz and feldspar in the concentrate have a great significance for the formation of the vitreous phase in the fired sample.

Creep of porcelain-containing silica and alumina

Hard porcelain ceramics find many applications because of their high hardness, high mechanical strength and moderate thermal-shock resistance. The addition of alumina as a filler to porcelain increases its strength at room temperature. In the present investigation, four-point-bend creep tests were carried out for porcelain-containing silica (SP-1) and alumina (AP-3) at 800, 900 and 1000°C. The creep data were analysed using a power-law creep, and the stress exponents were estimated. The activation energy for these two materials was found to be 45 kcal mol−1. The viscosity of the feldspar glassy phase was also determined from the creep tests. The test samples were analysed by scanning electron microscopy (SEM). The X-ray diffraction results (XRD) show that the amount of crystalline phase in the material increases after creep testing.

Cyclic Fatigue Behaviour of Porcelain under Repeated Impact

Ceramics are often put to applications where they are subjected to cyclic stresses frequently involving sudden loading. Thus, in the design of such materials there is growing concern about their response to repeated impact behaviour. In the present work, an improved pendulum type impact apparatus has been described for determining single and repeated impact strength. A definite fatigue behaviour has been demonstrated for specimens of a hard porcelain with progressive endurance at lower stress regimes. Endurance limit beyond 2 × 104 cycles was about 24.12% of the single impact breaking strength. Fatigue resistance parameter of the material was 5.52. Microstructural evidence showed that the fatigue crack path was mainly transgranular, controlled by porosity. Effect of cumulative residual stress is suggested to explain the fatigue behaviour of the porcelain.

Influence of grinding contamination on high-temperature phases of pyrophyllite

Abstract Phases formed at high temperatures from ground pyrophyllite in porcelain and agate mortars have been studied by X-ray diffraction. Cordierite (2MgO.2Al 2 O 3 .5SiO 2 ) as one crystalline phase appears in ground samples in a hard porcelain mortar after being treated above 1300°C, due to the reaction of enstatite (MgO.SiO 2 ), derived from the balls used, with mullite (mainly 3Al 2 O 3 .2SiO 2 ) and cristobalite (SiO 2 ) originating from the decomposed pyrophyllite sample during heating. After milling experiments using agate mortar and balls, the formation of cristobalite at lower temperatures, in ground as opposed to unground materials in hard porcelain, is attributed to contamination resulting from the mill and balls used.

Structural strength of hard porcelain with allowance for technological factors

1. The axial-compressive strength of porcelain specimens cut from piston-type components that had been subjected to plastic shape variation during drying is almost 60% lower than the same value for rods free from compressive forces. 2. It is established that the axial compressive strength of hard porcelain, which had possessed approximately 2% of moisture in its own composition prior to firing, exceeds by nearly 30% the strength of porcelain specimens cut from articles of the ball-mill-component type, which contain 16–18% of moisture prior to firing. In this case, hard-porcelain cylinders glazed once after drying to a moisture content of not more than 2% possess the maximum compressive strength (∼1150 MPa). This strength exceeds by 10 and 15% the temporary strength of unglazed and also thrice-glazed specimens, respectively. 3. It is shown that cylindrical shells with a ratio of wall thickness and length of cylindrical section to the radius of the median section of 0.09–0.11 and 5–6, respectively, the bearing capacity of which may be 40–60 MPa for a one-time loading by external hydrostatic pressure, can be fabricated from hard porcelain. 4. It is established that geometric shape imperfections significantly affect the magnitude of the failing compressive stresses with exhaustion of the bearing capacity of porcelain shells under uniform pressure. In cases where shells of proper circular shape, which can be provided by diamond grinding, are tested, the ultimate stresses are comparable to the ultimate strength of porcelain in axial compression.

Control of industrial noise using ceramic sound absorber

This paper introduces the acoustic characteristics of the sound absorptive ceramics and their application for outdoor barriers. The developed sound absorptive ceramics, a kind of porous plates, are produced by binding the particles of hard porcelain with glaze. They have excellent weather‐durability and water‐ and fire‐resistant characteristics in striking contrast with conventional sound absorptive materials, such as glass wool and rock wool. The sound absorption coefficient curves of the ceramic show the combined characteristics of resonant‐type and porous‐type absorbers. The ceramic sound barriers constructed for transformer and other equipments noise sources showed the superior effect of noise reduction than that predicted by Maekawa's formula. The ceramics are also applicable for the acoustic treatment of ventilating ducts, steam‐exhaust silencers, machine rooms, and barriers for roads and railways, etc.

Crazing Resistance of Hard Porcelain Ware

Crazing Resistance of Hard Porcelain Ware

Ceramic Properties of Hard Porcelain in Relation to Mineralogical Composition and Microstructure: IV—Thermal-shock Resistance and Thermal Expansion

Thermal-shock resistance of hard porcelain samples are dependent on the concentrations of the crystalline phases (mullite and quartz) and the glassy phase and also on the size of mullite crystals. Internal stresses originating in the samples from the thermal expansion mismatch between the phases also influence the thermal-shock resistance. The thermal expansion of the samples is a function of the asymmetry of size-distribution of mullite crystals.

Ceramic Properties of Hard Porcelain in Relation to their Mineralogical Compositions and Microstructures: II—Creep Characteristics

Creep of hard porcelains has been studied by a short time test, avoiding changes in the phase composition and minor trace during the test. The thermal activation energy of creep ranged from 121.4 to 277 Kcal/mole depending on the mineralogical composition and microstructure. The activation energy, which is a measure of the thermal sensitivity, has been found to be indirectly proportional to the concentration of mullite, directly to the concentration of the glassy phase and almost independent of the quartz content. The size of mullite crystals, however, has a stronger influence on the activation energy which is directly proportional to the size. The thermal activation energies of creep and the viscous flow of the glassy phase are almost same indicating thereby the strong influence of the glassy phase on the creep characteristics of the samples.

Mullite in Hard Porcelain

The type and size of mullite crystals formed in hard porcelain bodies have been studied by X-ray diffraction and electron microscopy. The average sizes measured by these techniques have been compared.