[0001] The present invention relates to a mechanical element, in particular to a component
for a combustion chamber of a high speed diesel engine, of the type having sliding
surfaces provided with wear-protection layers which are used up during the running-in
period, and to a process for obtaining such layers.
[0002] It is known that for the purpose of extending the life of diesel engines and the
maintenance intervals thereof it is sought to encourage a correct running-in by means
of the application of wear-protection layers on the sliding surfaces of the mechanical
elements forming components of the engine, and in particular those forming the combustion
chamber, such as the piston rings, cylinder sleeve, valve stems, valve seats, hollow
pistons (in spheroidal or laminate cast iron, or steel), gudgeon pins, injectors and
injection pumps. The protective layers currently used are constituted by electrolytically
deposited layers of copper and/or tin, layers of phosphates and iron oxides produced
by chemical reaction, or by organic or inorganic anti-friction layers (for example
cresol resin with graphite).
[0003] The known wear-protection layers described above have the disadvantage of becoming
rapidly destroyed so that they facilitate running-in only during the first hours of
operation, whilst the running-in period necessary for engines can be up to even about
2000 hours.
[0004] The object of the present invention is to provide mechanical elements having sliding
surfaces coated with wear-protection layers of high thermal stability, of high resistance
to oxidation and corrosion, even in the presence of gas containing sulphur, and having
low friction and long life.
[0005] The said object is achieved by the present invention in that it relates to a mechanical
element forming a component of a combustion chamber of a diesel engine in particular,
of the type comprising at least one sliding surface coated with a wear-protection
layer which can be worn away during a running-in period, characterised by the fact
that the said wear layer is constituted by a nitride based heat diffusion layer containing
a percentage by weight of nitrogen lying between about 4% and 12%.
[0006] The present invention further relates to a process for forming a wear-protection
layer over a sliding surface of an element forming a component of a combustion chamber
of a diesel engine, which can be worn away during the running-in period of the said
engine, characterised by the fact that it comprises a phase of gaseous nitriding of
the said surface performed with ammonia or nitrogen ionised at a temperature less
than or equal to 600°C in conditions such as to obtain on the said surface the formation
of a layer of nitrides substantially of 6 - type.
[0007] For a better understanding of the present invention a non limitative description
of an embodiment thereof will now be given with reference to the attached drawings,
in which:
Figure 1 schematically illustrates a portion of a mechanical element provided with
a sliding surface coated with a protective layer formed according to the principles
of the present invention; and
Figures 2 and 3 are qualititive wear diagrams illustrating the wear of an element
protected with the layer of Figure 1 and the same element protected with a conventional
wear-protection layer respectively.
[0008] With reference to Figure 1 there is indicated a mechanical element 1 of any type
having at least one sliding surface 2 cooperating frictionally with a similar sliding
surface of another element; the element 1 can be constituted by any member forming
a component of an engine and in particular by a member forming a component of or involved
in the combustion chamber of a diesel engine, such as a piston ring, cylinder sleeve,
valve stem or valve seat, a hollow piston for housing the piston rings (both pistons
made of spheroidal or laminar cast iron, and those of steel), a gudgeon pin or an
element of an injector or an injection pump.
[0009] The present invention can find application, equally, in one or all of the mechanical
members listed above, and in particular to the piston rings which are among the mechanical
elements most subjected to wear in an engine, and which have, as is known, three sliding
surfaces 2 particularly subjected to wear, and constituted by the outer lateral surfaces
thereof intended to cooperate by sliding against the wall of the cylinder sleeve to
exercise a sealing action against the combustion gases on one side and the lubricating
oil on the other, and by the upper and lower surfaces intended to engage against the
walls of the hollow piston in which the piston ring is housed.
[0010] The said lateral surface 2 of the piston ring is preferably coated, as is illustrated
in Figure 1, by a plasma sprayed layer 3 of a metal alloy and/or metal/ ceramic (for
example based on Cr-carbide or Mo-carbide, Cr and Nicr) of great hardness substantially
greater than that of the material constituting the element 1; in this case the surface
2 does not work directly but serves solely as a support for the layer 3 an outer surface
2a of which constitutes the actual sliding surface of the element 1.
[0011] In either case, according to the invention, the sliding surface 2 or 2a is coated
with a wear-protection-layer 4 which can be worn away during a running-in period of
the element 1, in such a way as to expose the surface 2 (or 2a) of great hardness
at the end of the running in period.. The layer 4 must have low friction and be relatively
soft, of hardness substantially equal to or only slightly greater than that of the
material from which the element 1 is made, and must at the same time have a hardness
and a resistance to wear such as to allow it to last for a period at least equal to
the running-in period of the element 1, which in the case of piston rings or other
elements forming components of an engine, is of the order of 1000 hours of operation.
Such characteristics are obtained, according to the invention, with wear layers 4
constituted ty nitride based heat diffusion layers containing a percentage by weight
of nitrogen lying between about 4% and about 12%, preferably being iron, chrome and
molibdenum elements which give, with the nitrogen, nitrides having a quite satisfactory
degree of heat stability, and with the element 1 being made of an alloy containing
such elements (steel or cast iron), the layer 4 is based on nitrides of iron (and/or
chrome or molibdenum) of the chemical formula Fe
xN where x is a number, even a decimal, the value of which must lie between two and
four, and in particular, must be greater than or equal to two and less than four.
Such chemical compounds are well known in the art under the name "white sheet" by
which are meant solid E and ε type solutions of nitrogen in iron having compact hexagonal
and rhombic structures respectively, and with nitrogen contents lying between 4% and
12%. At the maximum degree of nitriding the ε phase corresponds with a good approximation
to the formula Fe
2N and, at a lower egree of nitriding, to the formula Fe
3N,
[0012] As is known, ε nitrides or "white sheet" are obtained as unwanted by-products of
both liquid and gaseous conventional hard nitriding of mechanical elements, the object
of which is to harden the surface thereof by the formation of an intermetallic γ compound
of the formula Fe
4N.
[0013] The principal known utilisation of ε nitrides has until now consisted in the preparation
of layers of such nitrides by means of nitriding operations conducted at high temperatures
(close to about 700°C)against the 590°C or a normal nitriding) on surfaces of elements
subjected to corrosion by moisture, for the purpose of forming protective e nitride
layers which protect the elements against corrosion; in fact it was discovered that
ε nitrides in saline solutions had cathodic potentials (about +0.10v NHE) which made
them substantially impervious to attack by corrosive agents (protective nitriding).
[0014] Now, after numerous experimental tests, the applicant has surprisingly found that
E nitrides or "white sheet" particularly those of iron, have low coefficients of friction,
high resistance to oxidation and high thermal stability, and an excellent resistance
to wear in the particular chemical and physical conditions present in the combustion
chamber of a heat engine, in particular both high speed and slow diesel engines, even
in the presence of low quality fuels which generate sulphurous combustion gases rich
in fuel ash. Comparative wear tests performed on piston rings provided with conventional
electro-deposited wear layers of copper and tin and on piston rings provided with
wear layers constituted by ε nitrides of iron have provided results set out in the
semi-logarithmic diagrams of Figures 2 and 3; in particular in Figure 2 there is illustrated
a qualititive diagram which plots the variation of wear (W) of a piston ring according
to the invention with respect to a reference parameter β (preferably time in hours
of operation, or else distance in Km travelled), whilst in Figure 3 there is illustrated
a similar diagram which plots the variation of wear (W) of a conventional piston ring
provided with an electro-deposited wear layer, with respect to the same reference
parameter β. As can be seen from Figures 2 and 3 the said diagrams are each sub-divided
into three sections, respectively a,b and c, relating, in order, to the running in
period, to the main lifetime period of operation of the element, and to the end period
in which collapse by wear takes place; the values of β are plotted on a logarithmic
scale. With reference to Figure 2, for the whole of the section a of the associated
diagram a substantially parabolic variation of W can be seen, which is free from discontinuities;
at the end of the section a (about 1000 hours of operation) the variation of W is
such as to join without discontinuity with the rectilinear section b. This corresponds
to a lifetime of the layer 4 substantially equal to the running-in period of the element
1. On the other hand, in Figure 3 the section a has a discontinuity and is composed
of a first curved section with strong inclination, corresponding to a period β of
several tens of hours, and by a rectilinear section which constitutes an extension
of the section b; this corresponds to a duration of the electro-deposited protection
layer of several tens of hours only, compared with the running-in period of a thousand
hours which involves, as can be seen by comparison of the two diagrams, a shorter
overall lifetime of the element (the reduction of the lifetime is equal to the quantity
α ), and a greater wear in the running-in period. The same comparative tests have
moreover permitted a significant reduction to be detected in the value of the coefficient
of friction of the wear-protective layer according to the invention with respect to
that of conventional electro-deposited layers.
[0015] The layer 4 of nitrides is obtained, according to the invention, by means of a particular
gaseous nitriding treatment at low temperature (less than or equal to 500°C) performed
in nitriding ovens in the form of-autoclaves, in which the nitriding is performed
by means of a gaseous fluid of ammonia (NH
3) or ammonia mixed with methane (CH
4) at an aboslute pressure lying between one and ten Torr (one Torr is approximately
equal to 1 mm of mercury
0.0013 atmospheres). In such ovens, in a known method, but applied to different technical
fields, the elements to be nitrided are maintained at cathodic potential, whilst the
metal walls of the oven are maintained at an anodic potential; between the walls and
the elements there is then applied a potential difference sufficient to ionise the
gaseous atmosphere between the anode and cathode. According to the invention the potential
difference for performing the nitriding at the temperature indicated (less than or
equal to 500 C) must be equal to or greater than 350v. With temperatures lying between
about 350°C and 500°C pressures of one to ten Torr, potential differences of 380v
or more and nitriding times lying between two and thirty hours, diffusion layers
5 of nitrogen are produced in the surface 2 of about 0.1 to 0.5 mm in thickness (Figure
1). The layer 5 comprises three successive layers, increasingly rich in nitrogen the
further they are from the surface 2 (towards the outside) and in fact comprises the
outermost layer 4 constituted by E nitrides, followed by an immediately underlying
layer 6 of monophasic γ' nitride having a formula Fe
4N, followed in turn by a diffusion layer 7 having a low nitrogen content (nitrogen-ferrite
and nitrogen- austenite) similar to that which can be found on mechanical pieces subjected
to the known TENIFER liquid nitriding process.
[0016] The layers 6 and 7 are obtained as an involuntary and inevitable consequence of producing
the layer 4 and also involve the possible coating layer 3; it has been found that
such supplementary layers 6 and 7 underlying the wear layer 4 and covering the surface
2, further improve both the mechanical characteristics and the resistance to wear
of the element 1; in fact the layer 7 increases the resilience and the resistance
to fatigue of the element 1 with an action similar to that of the diffusion layers
which can be obtained with the TENIFER process, whilst the layer of rt nitrides of
thickness lying between two and fifteen micron and of much greater hardness than that
of the base material of the element 1 , protects the surface 2 from fretting corrosion
after the wear layer 4 has been used up at the end of the running-in period, allowing
possible coating layers of electro-deposited chrome to be dispensed with. In the case
of a plasma coated layer 3 being present (based on chrome- molibdenum alloys of great
hardness) this is also produced by introducing a percentage of iron into its composition
in such a way that it can also form iron nitrides within it, whereby to obtain, simultaneously
with the the formation of the layer 4, an increase in the hardness and mechanical,
chemical and physical characteristics of the coating layer 3 and consequently of the
sliding surface 2a.
[0017] Finally, thanks to the particular nitriding process used, it is possible to work
at a temperature not greater than 500°C, thereby avoiding possible deformations of
the element 1; this is particularly important for the piston rings which are easily
subject to thermal deformations, for which a hard nitriding treatment, or worse, a
"protective nitriding" of conventional type would be entirely unsuitable because of
the high temperatures at which such treatments are performed.
1. A mechanical element (1) forming part of a combustion chamber of a diesel engine,of
a type comprising at least one sliding surface (2,2a) coated with a wear-protection
layer (4) which can be worn away during a running-in period, characterised by the
fact that the said wear layer (4) is constituted by a nitride based heat diffusion
layer containing a percentage by weight of nitrogen lying between about 4% and 12%.
2. An element (1) according to Claim 1, characterised by the fact that the said heat
diffusion layer is based on iron nitrides of chemical formula Fe N x where x is a
number greater than or equal to two and less than four.
3. An element (1) according to Claim 1 or Claim 2, characterised by the fact that
immediately beneath the said wear-protection layer (4) the said surface (2,2a) is
coated with an anti-wear layer (6) of single phase nitride of thickness lying between
two and fifteen microns, and a hardness greater than that of the material with which
the said element is made.
4. An element (1) according to Claim 3, characterised by the fact that the said anti-wear
layer (6) is constituted predominantly by iron nitride of formula Fe4N.
5. An element (1) according to any preceding Claim, characterised by the fact that
the said wear-protection layer (4) has a hardness substantially equal to or greater
than that of the material of which the said element (1) is made.
6. An element (1) according to any preceding Claim, characterised by the fact that
beneath the said wear-protection layer (4) the said surface (2) is coated with a plasma
sprayed layer (3) of a metallic or metal-ceramic alloy.
7. An element (1) according to any preceding Claim, characterised by the fact that
the said wear-protection layer (4) is obtained by means of a gaseous nitriding process
at a temperature equal to or less than 500°C.
8. An element (1) according to any preceding Claim, characterised by the fact that
it comprises a piston ring for a high speed diesel engine.
9. A process for forming a wear-protection layer (4) over a sliding surface (2,2a)
of an element (1) constituting a component of the combustion chamber of a diesel engine,
which can be worn away during the running-in period of the engine, characterised by
the fact that it comprises a gaseous nitriding phase of the said surface (2,2a) performed
with amomnia at a temperature less than 500°C in conditions such as to obtain on the
said surface (2,2a) the formation of a nitride layer substantially of e type.
10. A process according to Claim 9, characterised by the fact that the said nitriding
phase is performed at an absolute pressure lying between one and ten Torr and by applying
a potential difference between the said element (1), held at cathodic potential, and
a wall of a nitriding oven within which the said nitriding phase is performed, maintained
at anodic potential equal to or greater than about 350 volts.