www.ptcer.pl/mccm
JANSZ PARTYKA*, MARCIN GAJEK, KATARZYNA GASEK
AGH University of Science and Technology, Faculty of Material Science and Ceramics, Department of Ceramics and Refractory Materials, al. A. Mickiewicza 30, 30-059 Kraków, Poland
*e-mail: partyka@agh.edu.pl
1. Introduction
Glazes determine the most important usable and tech-nical parameters of ceramic products. These properties depend mainly on the quality of glaze surface. Two methods of correcting the glazes parameters, including surface quality, are functioning in the technology of manufacture of non-re-fractory ceramic goods. The fi rst of them is the modifi cation of the molar oxide composition, which relies on the knowledge of the properties of individual oxides and their effect on the glaze parameters [1-3]. However, this method is often unre-liable, as the role of oxides is only known for simple systems, whereas contemporary glazes are multi-component systems, in which the effects of either the synergy or weakening of the force of interaction between individual oxides exist [4-5]. The second method involves changing the fi ring parame-ters of glazed products by either increasing the maximum temperature or elongating the fi ring process. Such changes are diffi cult to accomplish, chiefl y due to economic reasons. Few examples of other techniques of modifying the surface properties of glazes can be found in literature. Methods of multilayered deposition of ceramic glazes are known. Two layers of raw glaze are applied onto the green body in two
Study of the topography and roughness of
the glaze surface as modifi ed by selection
of raw materials grain size
Abstract
The presented paper shows a new, simple method for improving the surface properties, such as smoothness, gloss and whiteness, of ceramic glazes. This method consists in making the intentional selective choice of the grain size of selected raw-material components. In the investigation, the variable factor was the grain size distribution of hard raw materials, that is quartz, feldspar and zirconium silicate. The obtained results indicate that the proper selection of the grain size of individual raw materials markedly improves the surface quality.
Keywords: Grain size, Glaze, Surface, Roughness, Whiteness degree
MODYFIKACJA TOPOGRAFII I CHROPOWATOŚCI POWIERZCHNI SZKLIWA POPRZEZ DOBÓR UZIARNIENIA SUROWCÓW
Prezentowana praca pokazuje nową, prostą metodę poprawiania właściwości powierzchniowych, takich jak gładkość, połysk i stopień białości szkliw ceramicznych. Metoda ta polega na świadomym selektywnym doborze uziarnienia wybranych składników surowcowych. W badaniach czynnikiem zmiennym było uziarnienie surowców twardych, czyli kwarcu, skalenia oraz krzemianu cyrkonu. Uzyskane wyniki pokazują, że odpowiedni dobór uziarnienia poszczególnych surowców znacząco poprawia jakość powierzchni.
Słowa kluczowe: rozmiar ziarna, szkliwo, powierzchnia, chropowatość, stopień białość
stages and then jointly fi red, or a thin layer of transparent glaze is applied onto the fi red coat of white opaque glaze, after which the product is fi red again [6]. None of the methods brings about the expected results, mainly due to the diffi culty in producing the uniform thickness of the applied coatings, but also because of the fact that the two glazes react with each other during fi ring. Studies on the effect of grain size distribution on the quality of glazes have been performed by Koenig and Henderson [7], Bernandin [9], and Danielson [10]. They concern the infl uence of variations in the duration of grinding in traditional ball mills on the wettability and reactivity between the glaze and body, as well as on the occurrence of defects, such as glaze crawling or rolling up.
Examination of the ceramic glaze surface is diffi cult. The high gloss of glazes, or their full or partial transparency, make the determination of even their colour and shine diffi cult. Since recently, the study of the topography of fi red glaze surfaces has been enabled to be fully carried out owing to the development of microscopic techniques. The techniques most commonly used for this purpose include: AFM electron microscopy and the confocal laser microscopy technique [12-14]. The second technique in particular deserves spe-cial attention, as it enables satisfactory surface topography
images and surface fi nish results to be obtained in an non--invasive technique and with no special sample preparation.
2. Experimental procedure
Opaque glaze for a fi ring temperature of 1210-1220 °C, with molar composition as shown in Table 1, was used for testing. The raw materials used included quartz, two feldspars (a mixed alkaline oxide and a potassium oxide types), zirconium silicate, wollastonite, talc and kaolin. The chemical analysis of the raw materials is given in Table 2. The preparation of glaze sets involved regrinding of indivi-dual raw-material groups in a MicrCer laboratory bead mill supplied by Netzsch. Quartz, which was classifi ed to the fi rst raw-material group, was ground down to four different grain sizes. The second group included both feldspars, which were jointly ground down to three grain sizes. Zirconium silicate was used both in the commercial form and as ground in the MicroCer mill. The remaining raw materials, i.e. kaolin, wolla-stonite and talc, were ground jointly in a Gabbrielli planetary mill for a duration of 15 minutes. Particle size distributions of raw materials were determined by an X-ray analyzer Sedigraph 5100 from Micromeritics. The grain size para-meters of all raw-material groups are presented in Table 3. Individual groups were combined as per the recipe (Table 1) and homogenized in the planetary mill for 10 minutes. The grain size composition of all glazes are shown in Table 4. Homogenization was conducted in the planetary mill with a small addition of grinding media to avoid any change in the grain size distribution of the components, but only to achieve a high degree of homogeneity. The glaze suspensions, im-mediately prior to application, were additionally homogenized for 10 minutes using ultrasounds. The above-mentioned methodology was employed to take control over the grain size distribution of the glazes. The glaze, in the amount of 6 grams, was applied onto disks of 50 mm in diameter made of Vitreous China sanitary body from Sanitec Company (preliminarily bisque fi red at a temperature of 1000 °C). Glazing was carried out using a standard spray gun. The test samples were fi red jointly in a laboratory electric furnace
Table 1. Molar composition of tested glaze.
CaO MgO ZnO 0.78 Al2O3 0.45 Na2O 0.22 SiO2 4.41 K2O ZrSiO4 0.26
Table 2. Chemical analysis of the raw materials.
Name Oxide composition [wt%]
SiO2 Al2O3 CaO MgO Na2O+K2O ZrSiO4 Feldspar 76.06 14.59 0.20 0.10 9.05 Feldspar 66.37 18.47 0.40 14.76 Quarz 99.70 0.30 Wollastonite 51.79 0.42 46.26 1.22 0.31 Talc 65.95 0.60 33.45 Zirkon silicate 5.44 0.76 0.35 93.45 Kaoline 59.30 39.73 0.09 0.14 0.74
Table 3. Characteristics of the grain size distribution of the raw materials used.
Raw materials
d50 [μm ]
Residue below grain size [%]
10 μm 1 μm 0.2 μm Quartz 8.33 58.6 5.0 0.0 1.44 89.6 16.5 5.6 0.37 92.5 23.6 18.6 0.27 94.6 86.4 41.2 Feldspars 3.99 76.7 16.2 1.1 0.43 99.4 91.6 24.2 0.15 96.0 96.3 59.5 ZrSiO4 1.37 99.6 36.8 11.7 0.25 99.2 94.9 37.4 Remaining raw materials 4.68 81.2 31.9 1.1
Table 4. Grain size compositions and surface parameters of glazes modifi ed by grain size selection.
Glaze name
Grain size glaze compositions d50 Surface parameters
Quartz Feldspar ZrSiO4
Remaining raw materials Whiteness L CIELAB Glossy [%]
Roughness: linear / surface defi ne in experiment [μm] [μm] GL-1 8.33 0.43 1.37 2.68 92.70 77.87 2.69 2.62 GL-2 1.44 93.59 80.35 0.73 0.82 GL-3 0.37 94.99 80.05 0.79 0.52 GL-4 0.27 95.01 82.66 0.25 0.27 GL-3 0.37 0.43 1.37 94.99 80.05 0.79 0.52 GL-5 0.25 95.93 86.21 0.28 0.28 GL-6 8.33 0.15 1.37 92.70 79.58 0.60 0.66 GL-7 0.37 93.23 80.75 0.28 0.30 GL-7 0.37 3.99 1.37 93.23 80.75 0.28 0.30 GL-8 0.15 93.55 81.51 0.22 0.27
at a temperature of 1220 °C for a total duration of 14 hours, consisting of 7 hours heating, 60 minutes soaking and 6 hours cooling. The above sample preparation procedure was aimed at eliminating any additional factors favouring the formation of surface defects. On the glaze surfaces of the fi red sam-ples, the determinations of gloss and whiteness were fi rst made using a Elcometer 406L and Konica-Minolta CM-700d spectrophotometer respectively. The measurement results are shown in Table 4. Then using an OLYMPUS Lext 4000 confocal laser microscope, glaze surface examinations were
made to determine the linear roughness, Ra, and the surface
roughness, Sa. The roughness results given (Table 4) are
mean values obtained from scanning two 1.28 mm side squ-are squ-areas on each sample. The images of the topography of the examined modifi ed glaze surfaces are shown in Figs. 1-3 (of which Figs. 1-2 are two-dimensional and Fig. 3 is three--dimensional). In addition, the characteristic temperatures of the examined glazes were determined using a Misura HSM 3M high-temperature microscope, by heating glaze pellets on the top of an alumina stand until they melted. The values of the characteristic temperatures (of sintering, softening, the sphere, the halfsphere, and melting) are shown in Table 5.
3. Results and discussion
In the research procedure, emphasis was laid on sys-tematic examinations based on the variability of a single factor only. Constant molar composition, the identical and averaged grain size distribution of each raw-material gro-up, the consistent glaze preparation procedure, the same amount of glaze applied on the biscuit and the joint fi ring process, ensure the correctness of the scheduled methods of research. Any changes of the surface parameters result only from the pre-planned modifi cations to the grain size distributions (Tables 3 and 4). A general conclusion drawn from the comparison of the surface parameters: brightness, gloss and roughness is unequivocal. Selective modifi cation to the grain size distribution of individual glaze components
Table 5. Characteristic temperatures of glazes modifi ed by grain size selection.
Glaze name
Temperature
Softening Sphere Half sphere Melting
[°C] GL-1 1178 1288 1326 1373 GL-2 1175 1287 1325 1371 -3 -1 -1 -2 GL-3 1166 1273 1322 1369 -12 -15 -6 -4 GL-4 1155 1268 1315 1354 -23 -20 -11 -19 a) b) c) d) e)
Fig. 1. Images of the surface of glazes modifi ed by grain size selection, a) GL1/(quartz d50 – 8.33 μm); b) GL2/(quartz d50 – 1.44 μm); c)
GL3/(quartz d50 – 0.37 μm, ZrSiO4 d50 – 1.37 μm); d) GL4/(quartz d50 – 0.27 μm), e) GL5/(ZrSiO4 d50 – 0.25 μm)), produced using a LEXT
may signifi cantly enhance the surface quality of the fi red glazes. The description of the glazes is based on providing the value of the median grain size, d50, of the raw material
being characteristic of a given glaze (quartz, feldspars or zirconium silicate, respectively).
By reducing the grain size of quartz from the value of d50
= 8.33 μm down to the fi nest grain size of d50 = 0,27 μm, at
a constant feldspar grain size of d50 = 0.43 μm, an almost
tenfold reduction in roughness, and an enhancement of whi-teness by 2,5% and gloss by 5% were achieved. In contrast, reducion of the particle size of the quartz from d50 = 1.44 μm
to d50 = 0.27 μm, at the constant grain size of feldspars d50 =
0.43 μm, leads to the surface roughness which is decreased approximately 3 times with a small improvement of whiteness and gloss (Table 4).
The surface parameters are also greatly infl uenced by the grain size of zirconium silicate. With a low grain size of quartz of d50 = 0.37 μm and feldspar of d50 = 0.43 μm, the
reduction of ZrSiO4 grain size from the average value of 1.37
μm (commercial powder) to 0.25 μm reduces the roughness almost by three times and considerably (by 6%) improves the gloss (Table 4).
The least signifi cant effect on the surface parameters of glazes is shown by the change in feldspar grain size. Table 4 indicates that with fi ne quartz of d50 = 0.37 μm used, reducing
the feldspar grain sized from d50 = 3.99 μm to d50 = 0.15 μm
results in a reduction in roughness by mere than 20%, with a slight improvement in gloss and whiteness.
The explanation of these phenomena can be grounded on the melting points of the basic components of the glaze. Pure feldspars, the sodium and potassium types, melt at temperatures of 1118 °C and 1150 °C, respectively, and their grain size distribution can only infl uence the melting kinetics. The liquid phase in aluminosilicate systems starts to form at a temperature slightly above 1000 °C due to the presence of feldspar-quartz eutectics, or even below 1000 °C in the presence of iron oxide, Fe2O3, which occurs in raw
mate-rials as an impurity. In this case, the grain size distribution, particularly of quartz, may be of signifi cance for accelera-ting the melaccelera-ting. Similar phenomena might occur at higher temperatures during the dissolution of quartz particles in the aluminosilicate melt formed by molten feldspars. The quartz grain size distribution may also have a signifi cant effect on the initiation and kinetics of this process. This is confi rmed by the characteristic temperature determination results presented in Table 5 for the glazes from GL1 through GL4. In these glazes, the average quartz grain size is changed from 8.33 μm for GL1 to 0.27 μm for GL4. The differences in characteristic temperatures between the glazes to which the coarsest and the fi nest quartz was introduces range from 11 °C to 23 °C. A change in characteristic temperatures may also indicate a reduction of glaze viscosity at the maximum fi ring temperature, which means a better glaze spreading
a)
b)
Fig. 2. Images of the surfaces of glazes modifi ed by grain size selec-tion, a) GL7/(feldspar d50 – 3.99 μm); b) GL8/(feldspar d50 – 0.15 μm)),
produced using a LEXT 4000 confocal microscope.
a)
b)
c)
Fig. 3. 3D images of the surfaces of glazes modifi ed by grain size selection, a) GL6/(quartz d50 – 8.33 μm); b) GL7/(quartz d50 – 0.37
μm, feldspar d50 – 3.99 μm)), c) GL8/(feldspar d50 – 0.15 μm)),
and easier elimination of surface defects. All of these factors favour the formation of smoother glazes of lower roughness, as confi rmed by the results presented in the paper.
4. Conclusions
The presented results allow us to drow the following conclusions:
– selective grinding and choosing the grain size distribution of hard raw materials, especially quartz and feldspars, make it possible to effect some surface properties of ceramic glazes;
– this method allows the degree of surface defecting to be reduced to a large extent;
– the greatest effect on the change in the surface parame-ters of glazes is exerted by quartz grain size reduction; – the effect of enhancing the surface quality of glazes
re-sults primarily from the change in glaze behaviour in the fi ring process, namely the reduction of the characteristic temperatures and viscosity;
– the increasingly common use of high-energy fl ow mills allows this method to be used in a tailor-made manner to optimize the manufacturing costs and product quality.
Acknowledgements
The study has been carried out within the framework of Research Project No. N N508 477734 fi nanced by the National Research and Development Committee (NCBiR).
The method is protected by the polish patent law, under Patent Application No. PL 390244 A1.
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