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Effects of spraying parameters onto flame-sprayed glaze coating structures
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Surface & Coatings Technology 202 (2008) 4444 – 4448 www.elsevier.com/locate/surfcoat
Effects of spraying parameters onto flame-sprayed glaze coating structures A. Arcondéguy a , G. Gasgnier b , G. Montavon a,⁎, B. Pateyron a , A. Denoirjean a , A. Grimaud a , C. Huguet b a
SPCTS — UMR CNRS 6638, Faculty of Sciences, University of Limoges, Limoges, France b Imerys Tableware France, Aixe-sur-Vienne, France Available online 9 April 2008
Abstract Thanks to their design characteristics (i.e., colors, brightness, opacity, etc.) and/or physical properties (i.e., durability, low thermal conductivity, tightness, etc.), glazes find numerous applications, from art ornamenting to protection against corrosion. Glazing consists in coating a substrate by fusing various mineral substances over it. This is a low cost process and hence can be applied on large surfaces. Conventional glazing process needs a relatively high temperature treatment (i.e., up to 1400 °C) that heat-sensitive substrates do not sustain. Thermal spraying may be a good solution to prevent the substrate from thermal degradation. Flame spraying was considered as the spray technique due to its low operating cost and the possibility to adapt the glaze transition temperature to the operating parameters. When spraying glazes, the coating formation mechanism is different from the one encountered with crystallized ceramic materials. Indeed, the high surface tension of those feedstock prevents the particles from being totally spread (i.e., “dewetting” phenomena). Here, the coating results from the coalescence of impinging particles to form a monolayer. The effects of glaze morphology on coatings were studied in this paper. Chemical analysis also permitted to determine the influence of spray parameters on glaze compositions, that can affect glazes thermal properties and hence modify coating structures. At last, the effects of operating parameters on coating architecture were analyzed by experimental design. © 2008 Elsevier B.V. All rights reserved. Keywords: Feedstock architecture; Feedstock composition; Flame spraying; Glaze; Operating parameters; Thermally sensitive substrate; Thick layer
1. Introduction Glazing can be depicted as coating a metallic or ceramic substrate by fusing various mineral substances over it. Conventional glazing process needs a relatively high temperature treatment (i.e., from 500 °C to up to 1400 °C in some cases) that some substrates do not sustain. Developing a glaze deposition technique by thermal spraying so may appear interesting as it could prevent, or at least limit, the substrate material from thermal degradation. Glazes are mainly made up of silica and alumina, which are refractory materials (i.e., high melting points): the relatively high fraction of silica in glazes makes them low thermally conductive. Furthermore, glazes present a relatively low viscosity when molten, which can affect the coating formation compared to spraying of more conventional ceramic materials [1,2]. Some previous studies showed that feedstock characteristics (powder morphology, particle size ⁎ Corresponding author. Tel.: +33 5 55 45 75 55; fax: +33 5 55 45 72 11. E-mail address: [email protected] (G. Montavon). 0257-8972/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.04.024
distribution) widely affect the coating morphology [3]. Other ones proved that an addition of glass (with a transition temperature of 610 °C) to mullite increased the coating porosity from 3 to 12% (Low Pressure Plasma Spraying onto molybdenum sheets) [4]. Furthermore, coatings properties depend upon spray parameters: increasing power levels and spray distance lead to an increase in coating porosity and tensile strength [3]. This paper intends to present some developments carried-out to process glaze feedstock by flame spraying. At first, the deposition process is presented and the characteristics of the glaze feedstock are recalled. Effects of selected operating parameters on coating structure for a given glaze composition are then studied by experimental design. 2. Experimental protocols 2.1. Feedstock powders Detailed chemical compositions of glazes specifically elaborated are confidential. Glazes powders are manufactured
A. Arcondéguy et al. / Surface & Coatings Technology 202 (2008) 4444–4448
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Fig. 1. Evolution of glaze composition according to thermal treatment (ICP-AES).
by agglomeration and sintering (Imerys Tableware France, Aixe-sur-Vienne, France). A connected porosity (~ 20%) is clearly discernable within powder particles. Previous studies [5] showed that such porosity rates are prejudicial for thermal spraying, as it induces high levels of globular pores in coatings. That is why densification by flame treatment was achieved and the treated particles were collected in water. After decantation, feedstocks were dried (24 h at 80 °C). This treatment is efficient: particles are spherical with a porosity level of about 5%. Particles densification when processed through the flame is confirmed by the measured particle size distribution: the two distributions are monomodal, nearly Gaussian, centered on 62 µm for as-received feedstock and 60 μm for the flame-densified one. Furthermore, the particle size distribution after the flame treatment is narrower than the as-received powders particle size distribution (d10 = 39 and 43 μm and d90 = 90 and 82 μm for as-received and densified feedstock, respectively). Consequently, such powders processed by flame treatment present an excellent flowability and show adequate compatibility to be used in flame spraying. As glazes are composed of various oxides, preferential vaporizations or decompositions could happen during the flame process (i.e., densification or/and spraying stages). Chemical composition analysis was carried-out by Inductive Coupled Plasma
Atomic Emission Spectroscopy (ICP-AES, Iris spectrometer, Thermo Fisher Scientific, Waltham, MA). Only the four principal oxides (#1, #2, #3 and #4) that play a relevant role on the softened glaze viscosity were titrated. Measured oxides losses are nearly the same as the ICP-AES experimental error (Fig. 1), which means that oxides are not vaporized nor decomposed when flame sprayed: glaze physical properties hence are not modified by flame treatments. 2.2. Substrates Substrates (40 × 40 × 15 mm) were made of a hydraulic binder whose composition and characteristics are confidential. They are thermally sensitive and material bursting might occur at temperatures of about 250 or 300 °C [6]. Furthermore, they present a porous structure and exhibit low thermal conductivity (~ 2 W m− 1 K− 1, supplier data). Substrates were dried (24 h at 50 °C) before spraying, to avoid water desorption during flame spraying, but no surface preparation was considered. 2.3. Test of operating parameters by experimental design A DS 8000 (CastoDyn, Lausanne, Switzerland) flame spray gun operated with a mixture of oxygen and acetylene was used
Table 1 Experimental matrix, from Plackett and Burman Experiments
1 2 3 4 5 6 7 8 (Reference)
Spray velocity (m s− 1)
Scanning step (mm per pass)
Spray distance (mm)
Feedstock rate (g min− 1)
Experimental responses Coating thickness (µm)
Coating porosity level (%)
0.150 0.150 0.150 0.070 0.150 0.070 0.070 0.070
3 1 1 1 3 1 3 3
90 90 150 150 150 90 150 90
30 20 20 30 30 30 20 20
1095 ± 58 754 ± 45 374 ± 34 585 ± 64 731 ± 95 875 ± 37 744 ± 73 1191 ± 34
10 9 18 15 19 9 16 20
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A. Arcondéguy et al. / Surface & Coatings Technology 202 (2008) 4444–4448
in this study. The total gas flow rate was kept constant to 50 SLPM. A stoichiometry of 0.6 (a common value used in flame spraying) was considered. From previous works [5], an operating parameters window has been identified. An experimental design from Plackett and Burman [7,8], corresponding to an additive model without factors matching, was considered to further optimize those operating parameters. Four factors were considered (Table 1): the feedstock feed rate (20 or 30 g min− 1), the spray distance (90 or 150 mm), the torch-substrate relative spray velocity (0.070 or 0.150 m s− 1) and the scanning step (1 or 3 mm per pass). 3. Results and discussion Mechanisms occurring during glaze coating flame spraying differ from those usually encountered when considering more traditional materials (oxides or alloys). Indeed, the coating results from the coalescence of molten particles and is manufactured in one pass rather than the stacking of individual lamellae and several passes [5]. Pores and voids are not connected together (Fig. 2): such layers might so be gas or liquid tight and might constitute a diffusion barrier against oxygen or water. To establish the effects of operating parameters on coatings structure, one chose to consider first two experimental responses: the coating thickness and
the coating porosity level, which are both estimated from polished cross-sections by image analysis (Table 1 and Fig. 2). Coatings thicknesses vary from 374 ± 34 μm to 1191 ± 34 μm, average value (from 8 measurements), according to operating parameters, with a variability varying from 3 to 13%. Coating average thickness is related to the feedstock rate impacting the substrate per time unit: this value depends indeed upon the torch-substrate relative velocity, the scanning step, the spray distance and the powder feed rate. Coating thickness is hence maximal for low velocity and scanning step combined to a high feedstock weight rate. Average effects of the different factors can be estimated by comparing the average responses for each modality factor (Fig. 3). In the two considered cases, the spray distance and scanning step exhibit the highest effects (contributions of 55% and 33% respectively). The effect of spray distance on the coating morphology can be explained by the fact that transferred flame flux clearly depends on spray distance [5]: the higher the thermal flux, the lower is the glaze viscosity, the better is the particle spreading and the lower is the coating porosity [3]. The effect of the spray distance is also due to the fact that radial feedstock distribution upon impact exhibits a Gaussian distribution: more softened particles impact the substrate for a lower spray distance. On the contrary, the torch-substrate relative velocity rate has a minor
Fig. 2. Polished cross-sections (SE-SEM) of glaze coatings obtained from the experimental design (×250 magnification). a) Exp. # 1. b) Exp. # 2. c) Exp. # 3. d) Exp. # 4. e) Exp. # 5. f) Exp. # 6. g) Exp. # 7. h) Exp. # 8.
A. Arcondéguy et al. / Surface & Coatings Technology 202 (2008) 4444–4448
4447
Fig. 3. Factor effects on a) coating thickness, b) coating porosity level.
effect on coating thickness and pore level (contributions of about 5 and 3%, respectively). The powder feed rate is not very influent too (contributions of about 1 and 10%, respectively). 4. Conclusion It is possible to manufacture glaze layers by flame spraying onto thermally-sensitive substrates and for which traditional glazing is not appropriate. The layers are crackfree and globular pores develop, very likely due to the coalescence of pores between the softened particles when they form the monolayer. Feedstock morphology can be improved by flame densification treatment. Chemical analyses show that glaze compositions remain constant during flame treatments (feedstock physical properties are not modified during the process). A Plackett and Burman experimental design permitted to highlight the effects of some major operating parameters on coatings morphologies. The spray distance is the most influent factor on coating thickness and pore level.
Acknowledgment This study is granted by the Association pour le Développement et la Promotion du Pôle Européen de la Céramique, Limoges, France, under grant number 05 005435/01/02. Its support is gratefully acknowledged by the authors. References [1] G. Aliprandi, (in French), Materiaux réfractaires et céramiques techniques — ingénierie des céramiques (Refractory Materials and Technical Ceramics — Ceramics Engineering), Pub. Septima Edition, Paris, France, 1979, p. 211. [2] T. Haure (in French), “Couches multi-fonctionnelles élaborées par plusieurs techniques” (Multifunctional Layers by Multitechnique Process), Ph.D. Thesis, Faculty of Sciences, University of Limoges, France, 2003. [3] E. Lugscheider, et al., in: C.C. Berndt, S. Sampath (Eds.), Thermal Spraying of Bioactive Glass Ceramics, Thermal Spray Science & Technology, Pub. ASM International, Materials Park, OH, USA, 1995, p. 583. [4] J. Disam, et al., in: C.C. Berndt, T.F. Bernecki (Eds.), Effect of Spraying Parameters of the LPPS Method on the Structure of Ceramics Coatings, Thermal Spray: Research, Design and Applications, Pub. ASM International, Materials Park, OH, USA, 1993, p. 487.
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A. Arcondéguy et al. / Surface & Coatings Technology 202 (2008) 4444–4448
[5] A. Arcondéguy, et al., Flame Journal of Thermal Spray Technology 16 (5–6) (2007) 978. [6] A. Saakov, et al., Protective-Decorative Plasma Coatings on Concrete Structures, Thermische Spritzkonferenz, Ed. DVS, Pub. German Welding Society, Düsseldorf, Germany, 1993, pp. 212.
[7] F. Louvet, (in French), “Plan de Plackett et Burman” (Plackett and Burman experimental design), Expérimentique, 2004, p. 1. [8] ISO TC 69/SC 1, ISO/FDIS 3534-3 (in French), “Statistique, vocabulaire et symboles, partie 3: plans d'expériences” (Statistics, Vocabulary and Symbols, Part 3: Experimental Designs), Ed. Genève, Paris, France, 1998, p. 34.
Surface & Coatings Technology 202 (2008) 4444 – 4448 www.elsevier.com/locate/surfcoat
Effects of spraying parameters onto flame-sprayed glaze coating structures A. Arcondéguy a , G. Gasgnier b , G. Montavon a,⁎, B. Pateyron a , A. Denoirjean a , A. Grimaud a , C. Huguet b a
SPCTS — UMR CNRS 6638, Faculty of Sciences, University of Limoges, Limoges, France b Imerys Tableware France, Aixe-sur-Vienne, France Available online 9 April 2008
Abstract Thanks to their design characteristics (i.e., colors, brightness, opacity, etc.) and/or physical properties (i.e., durability, low thermal conductivity, tightness, etc.), glazes find numerous applications, from art ornamenting to protection against corrosion. Glazing consists in coating a substrate by fusing various mineral substances over it. This is a low cost process and hence can be applied on large surfaces. Conventional glazing process needs a relatively high temperature treatment (i.e., up to 1400 °C) that heat-sensitive substrates do not sustain. Thermal spraying may be a good solution to prevent the substrate from thermal degradation. Flame spraying was considered as the spray technique due to its low operating cost and the possibility to adapt the glaze transition temperature to the operating parameters. When spraying glazes, the coating formation mechanism is different from the one encountered with crystallized ceramic materials. Indeed, the high surface tension of those feedstock prevents the particles from being totally spread (i.e., “dewetting” phenomena). Here, the coating results from the coalescence of impinging particles to form a monolayer. The effects of glaze morphology on coatings were studied in this paper. Chemical analysis also permitted to determine the influence of spray parameters on glaze compositions, that can affect glazes thermal properties and hence modify coating structures. At last, the effects of operating parameters on coating architecture were analyzed by experimental design. © 2008 Elsevier B.V. All rights reserved. Keywords: Feedstock architecture; Feedstock composition; Flame spraying; Glaze; Operating parameters; Thermally sensitive substrate; Thick layer
1. Introduction Glazing can be depicted as coating a metallic or ceramic substrate by fusing various mineral substances over it. Conventional glazing process needs a relatively high temperature treatment (i.e., from 500 °C to up to 1400 °C in some cases) that some substrates do not sustain. Developing a glaze deposition technique by thermal spraying so may appear interesting as it could prevent, or at least limit, the substrate material from thermal degradation. Glazes are mainly made up of silica and alumina, which are refractory materials (i.e., high melting points): the relatively high fraction of silica in glazes makes them low thermally conductive. Furthermore, glazes present a relatively low viscosity when molten, which can affect the coating formation compared to spraying of more conventional ceramic materials [1,2]. Some previous studies showed that feedstock characteristics (powder morphology, particle size ⁎ Corresponding author. Tel.: +33 5 55 45 75 55; fax: +33 5 55 45 72 11. E-mail address: [email protected] (G. Montavon). 0257-8972/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.04.024
distribution) widely affect the coating morphology [3]. Other ones proved that an addition of glass (with a transition temperature of 610 °C) to mullite increased the coating porosity from 3 to 12% (Low Pressure Plasma Spraying onto molybdenum sheets) [4]. Furthermore, coatings properties depend upon spray parameters: increasing power levels and spray distance lead to an increase in coating porosity and tensile strength [3]. This paper intends to present some developments carried-out to process glaze feedstock by flame spraying. At first, the deposition process is presented and the characteristics of the glaze feedstock are recalled. Effects of selected operating parameters on coating structure for a given glaze composition are then studied by experimental design. 2. Experimental protocols 2.1. Feedstock powders Detailed chemical compositions of glazes specifically elaborated are confidential. Glazes powders are manufactured
A. Arcondéguy et al. / Surface & Coatings Technology 202 (2008) 4444–4448
4445
Fig. 1. Evolution of glaze composition according to thermal treatment (ICP-AES).
by agglomeration and sintering (Imerys Tableware France, Aixe-sur-Vienne, France). A connected porosity (~ 20%) is clearly discernable within powder particles. Previous studies [5] showed that such porosity rates are prejudicial for thermal spraying, as it induces high levels of globular pores in coatings. That is why densification by flame treatment was achieved and the treated particles were collected in water. After decantation, feedstocks were dried (24 h at 80 °C). This treatment is efficient: particles are spherical with a porosity level of about 5%. Particles densification when processed through the flame is confirmed by the measured particle size distribution: the two distributions are monomodal, nearly Gaussian, centered on 62 µm for as-received feedstock and 60 μm for the flame-densified one. Furthermore, the particle size distribution after the flame treatment is narrower than the as-received powders particle size distribution (d10 = 39 and 43 μm and d90 = 90 and 82 μm for as-received and densified feedstock, respectively). Consequently, such powders processed by flame treatment present an excellent flowability and show adequate compatibility to be used in flame spraying. As glazes are composed of various oxides, preferential vaporizations or decompositions could happen during the flame process (i.e., densification or/and spraying stages). Chemical composition analysis was carried-out by Inductive Coupled Plasma
Atomic Emission Spectroscopy (ICP-AES, Iris spectrometer, Thermo Fisher Scientific, Waltham, MA). Only the four principal oxides (#1, #2, #3 and #4) that play a relevant role on the softened glaze viscosity were titrated. Measured oxides losses are nearly the same as the ICP-AES experimental error (Fig. 1), which means that oxides are not vaporized nor decomposed when flame sprayed: glaze physical properties hence are not modified by flame treatments. 2.2. Substrates Substrates (40 × 40 × 15 mm) were made of a hydraulic binder whose composition and characteristics are confidential. They are thermally sensitive and material bursting might occur at temperatures of about 250 or 300 °C [6]. Furthermore, they present a porous structure and exhibit low thermal conductivity (~ 2 W m− 1 K− 1, supplier data). Substrates were dried (24 h at 50 °C) before spraying, to avoid water desorption during flame spraying, but no surface preparation was considered. 2.3. Test of operating parameters by experimental design A DS 8000 (CastoDyn, Lausanne, Switzerland) flame spray gun operated with a mixture of oxygen and acetylene was used
Table 1 Experimental matrix, from Plackett and Burman Experiments
1 2 3 4 5 6 7 8 (Reference)
Spray velocity (m s− 1)
Scanning step (mm per pass)
Spray distance (mm)
Feedstock rate (g min− 1)
Experimental responses Coating thickness (µm)
Coating porosity level (%)
0.150 0.150 0.150 0.070 0.150 0.070 0.070 0.070
3 1 1 1 3 1 3 3
90 90 150 150 150 90 150 90
30 20 20 30 30 30 20 20
1095 ± 58 754 ± 45 374 ± 34 585 ± 64 731 ± 95 875 ± 37 744 ± 73 1191 ± 34
10 9 18 15 19 9 16 20
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A. Arcondéguy et al. / Surface & Coatings Technology 202 (2008) 4444–4448
in this study. The total gas flow rate was kept constant to 50 SLPM. A stoichiometry of 0.6 (a common value used in flame spraying) was considered. From previous works [5], an operating parameters window has been identified. An experimental design from Plackett and Burman [7,8], corresponding to an additive model without factors matching, was considered to further optimize those operating parameters. Four factors were considered (Table 1): the feedstock feed rate (20 or 30 g min− 1), the spray distance (90 or 150 mm), the torch-substrate relative spray velocity (0.070 or 0.150 m s− 1) and the scanning step (1 or 3 mm per pass). 3. Results and discussion Mechanisms occurring during glaze coating flame spraying differ from those usually encountered when considering more traditional materials (oxides or alloys). Indeed, the coating results from the coalescence of molten particles and is manufactured in one pass rather than the stacking of individual lamellae and several passes [5]. Pores and voids are not connected together (Fig. 2): such layers might so be gas or liquid tight and might constitute a diffusion barrier against oxygen or water. To establish the effects of operating parameters on coatings structure, one chose to consider first two experimental responses: the coating thickness and
the coating porosity level, which are both estimated from polished cross-sections by image analysis (Table 1 and Fig. 2). Coatings thicknesses vary from 374 ± 34 μm to 1191 ± 34 μm, average value (from 8 measurements), according to operating parameters, with a variability varying from 3 to 13%. Coating average thickness is related to the feedstock rate impacting the substrate per time unit: this value depends indeed upon the torch-substrate relative velocity, the scanning step, the spray distance and the powder feed rate. Coating thickness is hence maximal for low velocity and scanning step combined to a high feedstock weight rate. Average effects of the different factors can be estimated by comparing the average responses for each modality factor (Fig. 3). In the two considered cases, the spray distance and scanning step exhibit the highest effects (contributions of 55% and 33% respectively). The effect of spray distance on the coating morphology can be explained by the fact that transferred flame flux clearly depends on spray distance [5]: the higher the thermal flux, the lower is the glaze viscosity, the better is the particle spreading and the lower is the coating porosity [3]. The effect of the spray distance is also due to the fact that radial feedstock distribution upon impact exhibits a Gaussian distribution: more softened particles impact the substrate for a lower spray distance. On the contrary, the torch-substrate relative velocity rate has a minor
Fig. 2. Polished cross-sections (SE-SEM) of glaze coatings obtained from the experimental design (×250 magnification). a) Exp. # 1. b) Exp. # 2. c) Exp. # 3. d) Exp. # 4. e) Exp. # 5. f) Exp. # 6. g) Exp. # 7. h) Exp. # 8.
A. Arcondéguy et al. / Surface & Coatings Technology 202 (2008) 4444–4448
4447
Fig. 3. Factor effects on a) coating thickness, b) coating porosity level.
effect on coating thickness and pore level (contributions of about 5 and 3%, respectively). The powder feed rate is not very influent too (contributions of about 1 and 10%, respectively). 4. Conclusion It is possible to manufacture glaze layers by flame spraying onto thermally-sensitive substrates and for which traditional glazing is not appropriate. The layers are crackfree and globular pores develop, very likely due to the coalescence of pores between the softened particles when they form the monolayer. Feedstock morphology can be improved by flame densification treatment. Chemical analyses show that glaze compositions remain constant during flame treatments (feedstock physical properties are not modified during the process). A Plackett and Burman experimental design permitted to highlight the effects of some major operating parameters on coatings morphologies. The spray distance is the most influent factor on coating thickness and pore level.
Acknowledgment This study is granted by the Association pour le Développement et la Promotion du Pôle Européen de la Céramique, Limoges, France, under grant number 05 005435/01/02. Its support is gratefully acknowledged by the authors. References [1] G. Aliprandi, (in French), Materiaux réfractaires et céramiques techniques — ingénierie des céramiques (Refractory Materials and Technical Ceramics — Ceramics Engineering), Pub. Septima Edition, Paris, France, 1979, p. 211. [2] T. Haure (in French), “Couches multi-fonctionnelles élaborées par plusieurs techniques” (Multifunctional Layers by Multitechnique Process), Ph.D. Thesis, Faculty of Sciences, University of Limoges, France, 2003. [3] E. Lugscheider, et al., in: C.C. Berndt, S. Sampath (Eds.), Thermal Spraying of Bioactive Glass Ceramics, Thermal Spray Science & Technology, Pub. ASM International, Materials Park, OH, USA, 1995, p. 583. [4] J. Disam, et al., in: C.C. Berndt, T.F. Bernecki (Eds.), Effect of Spraying Parameters of the LPPS Method on the Structure of Ceramics Coatings, Thermal Spray: Research, Design and Applications, Pub. ASM International, Materials Park, OH, USA, 1993, p. 487.
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A. Arcondéguy et al. / Surface & Coatings Technology 202 (2008) 4444–4448
[5] A. Arcondéguy, et al., Flame Journal of Thermal Spray Technology 16 (5–6) (2007) 978. [6] A. Saakov, et al., Protective-Decorative Plasma Coatings on Concrete Structures, Thermische Spritzkonferenz, Ed. DVS, Pub. German Welding Society, Düsseldorf, Germany, 1993, pp. 212.
[7] F. Louvet, (in French), “Plan de Plackett et Burman” (Plackett and Burman experimental design), Expérimentique, 2004, p. 1. [8] ISO TC 69/SC 1, ISO/FDIS 3534-3 (in French), “Statistique, vocabulaire et symboles, partie 3: plans d'expériences” (Statistics, Vocabulary and Symbols, Part 3: Experimental Designs), Ed. Genève, Paris, France, 1998, p. 34.