ICMCTF1998 Session E3-2: Coatings Resistant to Erosion, Corrosion and Severe Environments Including Biomedical Environments (2)

Thursday, April 30, 1998 1:30 PM in Room Council/Chamber/Cabinet
Thursday Afternoon

Time Period ThA Sessions | Abstract Timeline | Topic E Sessions | Time Periods | Topics | ICMCTF1998 Schedule

Start Invited? Item
1:30 PM E3-2-1 The Effect of Nitrogen Pressure on Structure and Molten Aluminum Resistance of Arc Ion Plated Cr-N Coatings
K.Y. Yamamoto, F.Y. Yuse, T.S Satoh (Kobe Steel, Ltd., Japan)
A series of chromium nitride coatings with different nitrogen contents were deposited on Ti-6Al-4V substrate by the arc ion plating method using a Cr target and nitrogen as the reactive gas. At nitrogen pressures below 0.27 Pa, the deposited coatings contained 20 to 27 at% nitrogen and mainly consisted of the Cr2N phase. Above the pressure, coatings contained more than 35 at% nitrogen and consisted of the CrN single phase. The dissolution rate of the Cr-N coating in pure moltedn aluminum was drastically decreased above a nitrogen content of about 35 at%, which corresponds to the transition point from Cr2N to CrN. In TEM observation, it was found that a thin (~50nm) reactive layer confirmed to be as wruzite type AlN formed in the interface between the CrN and Al after Al immersion. It was concluded that the AlN acts as a protective layer for the molten aluminum. In heat cycle testing, formation of heat cracks was significantly suppressed when deposited on Ti-6Al-4V in comparison to deposition on conventional tool steel. This behavior agrees with the results of thermal stress calculations for both substrates.
1:50 PM E3-2-2 Corrosion Tests of PVD Coatings with Die Dressing Used for Al High Pressure Die Casting Dies
E. Lugscheider, C. Barimani, S. Guerreiro, K. Bobzin (Lehr- und Forschungsgebiet Werkstoffwissenschaften -RWTH- Aachen, Germany)

Under the extreme working conditions of the Al high pressure die casting process it is included the corrosion of the die surface, provoked by the die dressing sprayed on the hot die surface for better ejection of the casting piece. The major failure mechanism of the die is originated by the cyclic temperature change on the die surface, causing thermal fatigue on it. Although this failure is mainly caused by the temperature gradient between die surface and its bulk volume, it is expected a reduction on heat checking ocurrence for a die when its surface has good corrosion resitance, which results a smother surface.

PVD-coatings were deposited on hot work steel and corrosion tested with die dressing. Polarization curves, as current density potential, were taken for Titanium Aluminium Nitride and Cr based coatings, deposited by Magnetron Sputtering and Cathodic Arc processes. The coated and uncoated substrates were also tested after an oxidation in air at 720 C, which has shown better results due to the oxide formation on the coatings. SEM analysis was used for a better comparison of the results, which has confirmed the improvements achieved for the coated substrates.

2:10 PM E3-2-3 Friction and Wear Behavior of Self Lubricating Polymer/Steel Plasma Sprayed Coating Systems
M. Scholl (Oregon Graduate Institute of Science & Technology); D. Niebuhr (Quantum Corp.)
In many field applications, maintenance lubrication for friction reduction is expensive and often difficult. A self-sustaining lubricating film using solid lubricants requiring little or no maintenance would be an ideal solution. Adherence of the lubricant, endurance of friction reduction properties and ease of application are all factors in designing a friction reduction system. With these criteria in mind, a coating system based on a plasma sprayed steel coating followed by a plasma sprayed polymer coating was developed. Polymeric compounds have been used in many cases as a solid lubricant for friction reduction. Primary coating system variables include the thickness of the steel coating and the type of polymer. To test the practical application of such a designed coating system, an underlying steel coating was deposited using a high energy plasma spray (HEPS) system. A polymer coating, nylon, ultra high molecular weight polyethylene (UHMWPE), or polyvinylidene fluoride (PVDF), was then sprayed directly on top of the steel coating. To assess performance, the coated samples were tested under dry rolling/sliding contact in a twin roller testing machine over a range of contact pressures and creepages. Both durability and traction force (yielding the coefficient of friction) were measured during each test. The best overall behavior was found with a nylon/steel coating system. A simple explanation is also presented for the adherence and behavior of the polymer solid lubricant and it's interactions with the underlying steel coating.
2:30 PM E3-2-4 Characterisation of Unbalanced Magnetron Sputter Deposited Chromium Diboride Coatings
K.L. Dahm (The University of Auckland, New Zealand); P.A. Dearnley (University of Leeds, United Kingdom)

The synergistic, simultaneous action of corrosion and mechanical wear processes may incur more severe damage of components than that invoked by corrosion or wear alone. Such corrosive wear environments occur in many engineering applications. In spite of their large potential in such corrosive wear applications, little research has been conducted on chromium boride coatings obtained by physical vapour deposition. To redress this situation, CrB2 coatings were deposited on polished AISI 316L substrates by non-reactive unbalanced magnetron sputter deposition from a sintered CrB2 ceramic target. The substrate temperature during deposition remained below 200 degrees C.

X-ray diffraction studies showed that all of the coatings were crystalline with strong preferential orientation. In a previous study by the present authors an amorphous or quasi-crystalline structure was observed for coatings deposited from a ceramic CrB target. Nanoindentation hardness testing of the coatings showed that in all cases the coating hardness was greater than 43GPa (corresponding to approximately 4100HV) and exceeded the microhardness reported for bulk CrB2. The CrB2 coatings dramatically increased both the localised corrosion resistance (in NaCl, HCl and FeCl3 solutions) and general corrosion resistance (in H2SO4 solutions) of the substrates. Pin-on-disc wear testing showed that the wear resistance of the CrB2-coated samples may be limited by fracture of the brittle ceramic coating.

2:50 PM E3-2-5 On the Corrosion Behavior of TiN-coated AlSl D2 Steel
B.F. Chen, W.L. Pan, G.P. Yu, J. Hwang, J.H. Huang (National Tsin Hua University, ROC)
An extensive investigation on the corrosion behavior of the TiN-coated D2 tool steel was performed. Two series of experiment runs, one with the incorporation of Ti interlayer and the other without, were carried out in accordance with the experiment matrix of the Taguch's orthogonal Table L9, generating a total of 18 specimens. The study demonstrates that the corrosion properties of the TiN-coated D2 steel are primarily determined by the synergetic effect of the packing factor and thickness of the TiN coating. The packing factor reflects the denseness of a coating; whereas the coating thickness relates to the length of the diffusion path of corrosive media. Both of these factors have advantageous influences on corrosion protection as they are increased in magnitude. Hence, a minimum value of [packing factor]x[TiN thickness] would be required for an adequate protection against corrosion in a certain environment. This investigation has found that these minimum values are approximately 2.1 and 2.4 in a 5% NaCl solution and a 20% H3PO4 solution respectively. Above these critical values, the corrosion current densities of a TiN-coated D2 tool steel are practically nil. The synergetic effect of packing density and thickness of the coating is consistent with the results of a 500 hour salt spray test, which reveal that there exists a boundary separating the regime of corrosion immunity from that of corrosion attack in the [packing factor] versus [TiN thickness] plot. Therefore, the critical value of [packing factor]x[TiN thickness] can be used as a quick check for quality assurance on the corrosion protection of a TiN-coated metal in the industry. It was also demonstrated that a Ti interlayer of approximately 0.25 μm thick between the TiN coating and the metal substrate was useful in reducing the overall corrosion speed of the TiN-coated D2 tool steel, in addition to its being able to enhance the adhesion strength of the coating.
3:30 PM Invited E3-2-7 Hot Gas Erosion Resistance of a Vapor-deposited Cu-Cr Coating
K.T. Chiang (Boeing North American, Rocketdyne Division)
The effectiveness of a vapor-deposited Cu-Cr coating in protecting a NARloy-Z (Cu-3wt.%Ag-0.5wt.%Zr) substrate was investigated in a hydrogen/oxygen rocket engine test facility. The rocket facility generated high temperature, high pressure flows of combustion gas. The Cu-Cr coated NARloy-Z article was actively cooled by cryogenic gaseous hydrogen so that the coating surface temperature is maintained below 550°C. The coating provided good protection for the NARloy-Z substrate under both hydrogen rich and oxygen rich hot gas environments. In the erosion test (hydrogen rich), the coating remained predominantly a metallic mixture of Cu nd Cr phases. In the erosion/oxidation tests (oxygen rich), the coating surface formed a protective Cr203 scale under a thin layer of Cu-oxides. The ability of the coating to withstand repetitive thermal shocks from the hot gas impingement without cracking was attributed to the close match of thermal expansion properties between the Cu-Cr coating and the NARloy-Z substrate.
4:10 PM E3-2-9 Some Properties of Boronized AISI 440C and AISI 316L Stainless Steels
C.B. Bindal, I. Ozbek (Sakarya @um U@niversity, Turkey); M.U. Usta (Rensselaer Polytechnic Institute); S. Sen (Sakarya @um U@niversity, Turkey); A.H. Veprek (Bogazici @um U@niversity, Turkey)
In this study we investigated some properties of boronized AISI 440C stainless steel and AISI 316L austenitic surgical stainless steel. Boronizing was carried out in a solid medium consisting of Ekabor powder at a temperatur of 950°C for 2-8 h. The presence of borides e.g. Fe2B, CrB and Ni3B were revealed by means of x-ray diffractometer, scanning electron microscopy (SEM) and optical microscopes. The hardness of borides and boride layer thickness were measured via Vickers indentation technique. The hardness of borides formed on steel substrate was over 1500 VHN and it was observed that, the thickness of boride layers are strongly dependent on process time. It was also found that the longer boronizing time results in increase in thickness layer. Depending on the boronizing time, the thickness of boride layers ranged from 20 to 80 um. To determine distribution of alloying elements from surface to interior energy dispersive x-ray (EDX) was used.
4:30 PM E3-2-10 Chloride Induced Pitting Corrosion of Zirconium
F.A Malik (EMMAY Associates/National College, Pakistan); M. Hassan (EMMAY Associates, Pakistan)

Zirconium like other value metals (I.e., Ti. Al, Hf, Ta), forms a stable anodic oxide film which resists attach by mineral acids in a reducing atmosphere. In highly oxidising conditions, chlorides attach the zirconium oxide film.

Aggressive ions such as Cl, Br and F activate pits although the actual mechanisms are not fully understood. One mechanism suggests that Cl- ions chemisorb on cracks or pores in the passive layer and dissolve the film. Unaggressive ions such as So4--, NO3- and OH- sometimes decrease the attack by competitively absorbing on the metal surface.

The results of this investigation have been used to compare the corrosion of Zr with other metals. The pit nucleation potential (Ep) and the repassivation potential (Er) are the electro chemical parameters used to compare the pitting characteristics of Zr. Er has been determined primarily by the scratch method but limited results have also been acquired by using the potentiodynamic and potentiostatic methods.

4:50 PM E3-2-11 Galvanic Protection and Friction Properties of High Strength Steel Fasteners Sputter Coated with Aluminium or Zinc Alloys
E. Söderlund (Swedish Institute for Metals Research, Sweden); K. Algotsson (Saab Aerospace, Sweden); R. Carlsson (Bulten Automotive AB, Sweden); T. Hermansson (Volvo AB, Sweden); E. Hultgren (CSM Materialteknik AB, Sweden); R. Harrysson (Volvo Aero Corp, Sweden)
One of the usage of cadmium is for corrosion protection on high strength aircraft fasteners. In Sweden handling of cadmium today only can be made after authority dispensation. The Swedish government has recently suggested that a final date should be set for the existing dispensations. The suggested deadline is already in the beginning of the next decade and consequently there is now an urgent need of finding alternative coating materials. Any candidate material to replace cadmium must fulfil a number of critical requirements, such as galvanic compatibility with steel and aluminium, stable torque-tension properties and resistance to stress corrosion cracking and fatigue. In this project a number of sputtered aluminium and zinc alloy coatings on high strength steel fasteners have been evaluated with respect to their sacrificial protection of aluminium and steel as well as their resistance to de-icing solution. In addition, the friction of the coated fasteners have been studied.
Time Period ThA Sessions | Abstract Timeline | Topic E Sessions | Time Periods | Topics | ICMCTF1998 Schedule