Substrate and Reference Materials

Author:European Union Publications Office, 2006
Pages:19-26
SUMMARY

1. Reference Materials. 2. Materials Characterisations. Chemical composition of the stainless steels. Coating thickness. ESEM Surface observations. Surface structures. FTIR spectroscopy. Surface energy. Fingerprint resistance.

 
INDEX
CONTENT

Page 19

1. Reference Materials

Various types of materials have been selected and exchanged among the partners at the beginning of the Project as reference materials, in order to have common materials to coat with protective layers and to be compared to the new ones in term of damage resistance properties.

In particular, as regards the stainless steel substrate materials, three different substrates were chosen:

* 1.4301 (AISI 304), with bright surface (3D/2R)

* 1.4301(AISI 304), with brushed surface (Mikrolon)

* 1.4016 (AISI 430), with bright surface (3D/2R)

The aim was to consider both the influence of the composition and of the surface structure on the visibility of fingerprints and scratches in the uncoated and the coated condition and to investigate the influence of the different substrate surfaces on the adhesion to the coating.

In order to have a basis for the comparison of clear coats to be applied on stainless steels during the project, a material already in the market was chosen as coated reference material by DOC:

* 1.4301 (AISI 304), brushed surface (Mikrolon), coated with a 2.5 um thick clear coating named "Senocoil"

"Senocoil" is a thermal curing clear coat supplied by the German paint company Schramm. It is based on polyester and poly-butyl therephthalates. The substrate material was produced and the marketing was done by ThyssenKrupp Nirosta, whereas the organic coating was applied by ThyssenKrupp Stahl. However, the material was only introduced to the market to a small extent since its desiderative scratch resistance and chemical resistance were criticised by most of the customers from household appliances industry.

Another coated stainless steel was chosen (reference material for Arcelor and CSM):

* 1.4016 (AISI 430), brushed surface (Scotch Bright), coated with a PET film

This last material exists in the home appliances market since some years.

For single coating experiments, additionally also AISI 430 with brushed surfaces (Mikrolon or Scotch Bright) were used as substrate materials.

As regards carbon steel substrate, a commercial pre-painted coated steel for the domestic appliance industry was supplied by Voestalpine (figure 1-1). In addition to this two layer product, Voestalpine has chosen a further reference material, an experimental four-layer coating for possible automotive applications (figure 1-2). The reason was that the visual evaluation of scratched samples as well as gloss measurements revealed a relationship with the colour of the coating: the black (high gloss) reference material turned out to be most sensitive to the scratch tests applied at voestalpine and consequently the development of new coating systems concentrated on this type of material by varying the top clear coat (figure 1-2).

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The materials exchanged at the beginning of the Project are listed in the table 1-1.

Table 1-1: Reference materials


Description Alloy Finish Provided by
1 Uncoated stainless steel 1.4301 (AISI 304) 2R / Bright surface KTN
2 Uncoated stainless steel 1.4301 (AISI 304) Brushed surface (Mikrolon) KTN
3 Uncoated stainless steel 1.4016 (AISI 430) 2R / Bright surface KTN
4 Coated stainless stee 1.4301 (AISI 304) Brushed + clear coat Senocoil KTN
5 Coated stainless stee 1.4016 (AISI 430) Brushed + PET film Plalam (via Arcelor)
6 Coil coated carbon steel S250GD+Z HDG+ primer + topcoat CH26 (white) VASL

[ GRAPHICS ARE NOT INCLUDED ]

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2. Materials Characterisations
Chemical composition of the stainless steels

The stainless steels 1.4016 and 1.4301 with bright surfaces were analysed at DOC by means of a spectral analysis. The results are given in table 1-2. The 1.4016 is a ferritic chromium steel with minor fractions of Mn and Si. The 1.4301 is an austenitic chromium nickel steel with minor fractions of Mn, Si and Mo.

The composition of the surface-near regions of the different steel substrates were measured by means of secondary neutral mass spectrometry (SNMS) and x-ray photoelectron spectroscopy (XPS). On all surfaces (1.4016, bright; 1.4301, bright and brushed) oxides and, to a smaller extent, nitrides were found.

In the case of the bright 1.4016 enrichments of the less noble elements Cr, Mn and Si in the oxide / nitride form were found, whereas on the surfaces of 1.4301 a higher amount of iron oxides was detected.

Table 1-2: Composition of the stainless steels 1.4016 and 1.4301 as determined by means of spectral analysis. All values are given in wt. %


steel C Si Mn Cu Al Cr Mo Ni Ti Co
1.4016 (AISI 430) 0.03 0.3 0.35 0.05 17.0 0.18
1.4301 (AISI 304) 0.035 0.4 1.25 0.25 18.0 0.25 8.5 0.015 0.1

Coating thickness

The coatings thickness has been measured at CSM on the three coated reference materials (N° 4, 5 and 6) with magnetic or eddy currents transducers. Results are summarized in the following table:

Table 1-3: Thickness of the coatings on the reference materials


Description thickness (um)
4 AISI 304 Brushed + clear coat Senocoil ‹4
5 AISI 430 Brushed + PET film 26
6 HDG + primer + topcoat CH26 (white) 10+27

ESEM Surface observations

ESEM (Environmental Scanning Electron Microscopy) observations were made at Arcelor. The instrument uses a gaseous atmosphere between the sample and the detector as electron amplifier. This method has numerous advantages and permits us to have picture with a good resolution and to observe non-conductive or liquid sample without preparation. The observations of the six surfaces are given below. The brushed surface is visible with clear-coat and slightly hidden with the PET film.

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[ FIGURE ARE NOT INCLUDED ]

Surface structures

The structures of the substrate materials were determined at DOC by means of atomic force microscopy (AFM) measurements. Figure 1-4 represents the images of the stainless steel substrates. If one compares the two bright samples, it is clearly to see that the 1.4016 exhibits a smoother, more regular surface structure than the 1.4301. The brushed surface is, as to be expected, much rougher, and the grooves created by the brushing process are clearly to see. These visual impressions are confirmed by the roughness values which were evaluated using the images with areas of 86 um x 86 um. The values of the microscopic arithmetical mean deviation of the profile, Ra, are given in table 1-4. In figure 1-5 AFM images of the coated reference specimens are shown. The brushed stainless steel surface coated with Senocoil still exhibits grooves created by the brushing process, but the coating made them less deep and less sharp. This resulted in a decreased Ra value of 18.375 nm compared to the uncoated brushed surface (60.153 nm). This value is in the same range as the Ra values of the uncoated bright stainless steel surfaces (table 1-4).

Page 23

Table 1-4: Microscopic arithmetical mean deviations of the profiles, Ra, determined from AFM measurements on surface areas of 86 um x 86 um


Substrates Ra value [nm]
1 1.4301, bright surface 25.093
2 1.4301, brushed surface 60.153
3 1.4016, bright surface 18.855
4 1.4301, brushed surface + Senocoil 18.375

[ FIGURE ARE NOT INCLUDED ]

Page 24

FTIR spectroscopy

The samples 4, 5 and 6 have been analysed at Arcelor by FTIR spectroscopy in the following conditions:

- Apparatus : NEXUS (Thermo Optek)

- Accessory : ATR Golden Gate

- Detector : MCT

- Scans : 256

- Miror speed : 1.8988

- Resolution : 4cm-1

- Aperture : 100

The results are shown in the table 1-5 (see also spectra given in Annex 1).

Table 1-5: FTIR analysis of coated reference materials


Chemical nature Main stretching and bending frequencies
4 Aromatic Polyester with melamin binder (triazin) Aromatics : 3070-3050cm-1 (ν=CH), 1608cm-1 (νC=C),
726cm-1 (γhpC-H)
CH3 : 2964cm-1 (νasCH3), 1437cm-1 (γCH3), 1371cm-1
(γCH3)
Ester : 1715cm-1 (νC=O), 1227cm-1, 1096cm-1 (νC-O)
Binder : 1546cm-1 (νCNδNH), 813cm-1 (δtriazine)
5 Polyethylene terephtalate PET high gloss Aromatics : 3050cm-1 (ν=CH), 722cm-1 (γhpC-H)
CH2 aliphatic : 2963cm-1 (νasCH2)
Ester : 1711cm-1 (νC=O), 1244cm-1, 1095cm-1, 1017cm-1
(νC-O)
6 Aromatic polyester with melamin binder and pigments Aromatics : 3070-3050cm-1 (ν=CH), 726cm-1 (γhpC-H)
CH3 : 2953cm-1 (νCH3), 1472cm-1 (γCH3), 1371cm-1
(γCH3)
Ester : 1712cm-1 (νC=O), 1227cm-1, 1096cm-1 (νC-O)
Binder : 1546cm-1 (νCNδNH), 813cm-1 (δtriazine)
TiO2 (low wave length saturation)

Page 25

Surface energy

At DOC the surface energies of both the uncoated and the coated (ref. 4: Senocoil) stainless steels were determined. For this purpose, the contact angles between droplets of three liquids with different polarities (di-iodine-methane, ethylene glycol, water) and the surfaces were measured. Using an approximation according to Owen, Wendt and Kaelble, the polar and the disperse part of the surface energy could be calculated from these values. The total surface energy is the sum of these two parts. The results of these measurements are shown in figure 1-6.

[ GRAPHICS ARE NOT INCLUDED ]

The bright stainless steel surfaces show higher total surface energies than the brushed one. Whereas in the disperse parts no significant differences could be proven, the polar parts show higher values. The 1.4301 steel exhibits with 35mN/m by far the highest polar part of surface energy. This might be due to a treatment of the surface with acids in order to reach a high degree of gloss. The surface energy of the Senocoil coating is similar to the uncoated brushed 1.4301 surface.

Page 26

Fingerprint resistance

The observations of the six surfaces with natural fingerprint made at Arcelor with ESEM are given below.

Fingerprints appear different from one material to another.

[ FIGURE ARE NOT INCLUDED ]