Piston ring for internal combustion engines

Abstract

A piston ring 1 for internal combustion engines comprises a hard film 2 formed on at least an outer circumferential sliding surface of the piston ring 1. The hard film 2 includes chromium, nitrogen and silicon as structural elements and the same crystal structure as CrN, and is composed of a crystal phase where silicon is contained in a solid solution state in a crystal lattice at an atomic ratio between 1 and 9.5 percent. The hard film may be the following film. Namely, a hard film is composed of a mixed phase of a crystal phase and an amorphous phase; the crystal phase includes chromium, nitrogen and silicon as structural elements and the same crystal structure as CrN and moreover includes silicon contained in a solid solution state in a crystal lattice; the amorphous phase includes silicon, nitrogen and chromium as structural elements; the ratio of the amorphous phase in the hard film is 4.5 percent or less; and the silicon content in the hard film is between 1 and 9.5 percent at an atomic ratio. The hard film may include aluminum, vanadium, titanium, zirconium, boron, carbon, oxygen or fluorine.

Description

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a longitudinal sectional view showing a part of a piston ring of one preferred embodiment of the present invention;

FIG. 2 is a longitudinal sectional view showing a part of a piston ring of another preferred embodiment of the present invention;

FIG. 3 is a frontal view showing an outline of a reciprocating friction testing machine;

FIG. 4 is a frontal view showing an outline of a VDH testing machine;

DESCRIPTION OF PREFERRED EMBODIMENTS

The preferred embodiments of the present invention are described next. FIG. 1 is a longitudinal sectional view showing a part of a piston ring. A piston ring 1 is a ring with a rectangular cross-section and formed from steel, cast iron, titanium or titanium alloy, etc.

A hard film 2 covers the outer circumferential surface of the piston ring 1. The hard film 2 may be formed from any of the following structures.

(1) A hard film made up of chromium, nitrogen and silicon serving as structural elements, and having the same crystal structure as CrN, and moreover being composed of a crystal phase containing silicon in a solid solution state in a crystal lattice at an atomic ratio between 1 and 9.5 percent.

(2) A hard film made up of chromium, nitrogen and silicon serving as structural elements, and having the same crystal structure as CrN, and being composed of a mixed phase of an amorphous phase of silicon, nitrogen and chromium as structural elements and a crystal phase containing silicon in a solid solution state in a crystal lattice, wherein the ratio of the amorphous phase in the hard film is 4.5 percent or less, and the silicon content in the hard film is between 1 and 9.5 percent at an atomic ratio.

(3) A hard film according to the above aspect (1), wherein the hard film further includes one or more types of metallic elements selected from among aluminum, vanadium, titanium and zirconium as structural elements; and the one or more metallic elements are contained in a solid solution state in a crystal lattice, and the element content is 7 percent or less at an atomic ratio.

(4) A hard film according to the above aspect (2), wherein the hard film further includes one or more types of metallic elements selected from among aluminum, vanadium, titanium and zirconium as structural elements, and the one or more metallic elements are contained as the amorphous phase structural elements as well as being contained in a solid solution state in a crystal lattice of the crystal phase; and the element content is 7 percent or less at an atomic ratio.

(5) A hard film according to the above aspect (1), wherein the hard film further includes one or more types of elements selected from among boron, carbon, oxygen and fluorine as structural elements; and the one or more elements are contained in a solid solution state in a crystal lattice, and the element content is 10 percent or less at an atomic ratio.

(6) A hard film according to the above aspect (3), wherein the hard film further includes one or more types of elements selected from among boron, carbon, oxygen and fluorine as structural elements; and the one or more elements are contained in a solid solution state in a crystal lattice, and the element content is 10 percent or less at an atomic ratio.

(7) A hard film according to the above aspect (2), wherein the hard film further includes one or more types of elements selected from among boron, carbon, oxygen and fluorine as structural elements; and the one or more elements are contained as the amorphous phase structural elements as well as being contained in a solid solution state in a crystal lattice of the crystal phase; and the element content is 10 percent or less at an atomic ratio.

(8) A hard film according to the above aspect (4), wherein the hard film further includes one or more types of elements selected from among boron, carbon, oxygen and fluorine as structural elements; and the one or more elements are contained as the amorphous phase structural elements as well as being contained in a solid solution state in a crystal lattice of the crystal phase; and the element content is 10 percent or less at an atomic ratio.

The hard film 2 may be formed by PVD methods such as arc ion plating or sputtering. The method for forming the hard film of the above aspects (1) and (2) by the arc ion plating method utilizing a target formed from sintered chromium and silicon mixed compound is described next.

After degreasing and cleaning, a piston ring is placed on a rotation table within a film-forming chamber, and the air is exhausted to raise a vacuum. When the vacuum level reaches approximately 1.3×10⁻³ Pa, a heater inside the chamber heats the piston ring to approximately 673 K and the rotation table is driven simultaneously. The vacuum level temporarily weakens due to release of water vapor and gas components from the surface of the piston ring. When the vacuum level rises again to approximately 5×10⁻³ Pa, a small quantity of nitrogen gas is supplied, and an arc discharge occurs between the target and anode. The bias voltage of about −500 to −1,000 volts at this time is applied to the piston ring, and ions from the arc discharge strike the piston ring surface, performing the so-called bombard cleaning. This bombard cleaning raises the purity level of the piston ring surface, and enhances the adhesion of the hard film.

The bias voltage is then lowered at 0 to −100 volts approximately, and the supply of nitrogen gas is increased and the pressure in the chamber becomes 0.7 Pa to 4 Pa approximately, and the film is formed. The temperature of the piston ring during film-forming may be 723 K or higher in order to suppress the formation of an amorphous phase. Factors such as the arc current during film-forming or the film-forming time may be changed to control the thickness of the hard film. There are no particular specifications for the thickness of the hard film in the present invention but the thickness is preferably 5 micrometers to 50 micrometers approximately.

Prior to applying the hard film 2, an underlayer film 3 (See FIG. 2) of metallic chromium film or CrN film not containing silicon may be coated on the outer circumferential surface of the base material in the piston ring 1. In this case, besides a mixed cathode of chromium and silicon, a metallic chromium cathode is utilized, to first form an underlayer film of chromium or CrN, and the mixed cathode of chromium and silicon is then utilized to deposit the hard film of chromium, nitrogen, and silicon as structural elements. Forming this type of underlayer film improves the adhesion of the hard film.

Altering the silicon content of the target can change the silicon content in the hard film. However, the ratio of chromium to silicon within the target will not completely match the ratio of chromium to silicon within the fabricated film. The ratio of chromium to silicon within the film is usually a lower value than the ratio of chromium to silicon within the target. This lower value is due to the fact that silicon has a lower evaporating efficiency than chromium. The chromium within the target reacts with the nitrogen gas to form the CrN. The silicon within the target is contained in a solid solution state in a crystal lattice of the CrN or forms an amorphous phase containing silicon but does not form a particular compound.

Earlier, the amorphous phase was described as tending to be easily formed when there is excessive silicon content in the hard film. The amorphous phase also tends to be formed when an element such as aluminum is added or when the film-forming temperature is low. In other words, these methods can be utilized to change the percentage of the amorphous phase.

The silicon content in the hard film can be quantified by an electron beam microanalyzer or an X-ray fluorescence analysis apparatus.

When forming the hard films described in the aforementioned aspects (3) through (8) or in other words when purposely adding an element other than chromium, silicon and nitrogen to the hard film, the element may be added to the target or may be supplied as a gas. The elements aluminum, vanadium, titanium, zirconium and boron may be added to the target. The elements carbon, oxygen and fluorine may be supplied as a gas. If for example using carbon, then CH₄ gas, C₂H₄ gas or C₂H₂ gas may be utilized as a carbon source. If using oxygen, then oxygen gas may be utilized as an oxygen source. If using fluorine, then carbon tetrafluoride gas may be utilized as a fluorine source. The hard film may include elements other than described above as impurities in the above described PVD method.

Whether or not amorphous phase is present in the hard film can be investigated by X-ray diffraction measurement, electron diffraction measurement or high-resolution transmission electron microscope observation (lattice image observation). Observation by high-resolution transmission electron microscope is better for cases where there is little amorphous phase or when evaluating the ratio of the crystal phase to amorphous phase. In the present invention, the ratio of the crystal phase to amorphous phase is determined by finding the surface area ratio in images from high-resolution transmission electron microscope.

Tests were made to evaluate the wear resistance, crack resistance and peeling resistance of the hard film fabricated by the above described arc ion plating method.

The wear resistance test was performed using the reciprocating friction testing machine shown in FIG. 3. The reciprocating friction testing machine applies a load P from a spring load to press an upper test piece 11 equivalent to a piston ring, against a lower test piece 12 equivalent to a cylinder bore. The reciprocating movement of the lower test piece 12 induces a sliding motion between both test pieces. A tubing pump or an air dispenser supplied the lubricating oil. The wear length on the upper test piece 11 was measured with a surface roughness meter after operating the friction testing machine for a fixed amount of time at a specified load and speed. The wear length as the length of wear along the axial direction was found by the difference in contour of the lower test piece 12 before and after test operation. Test conditions were a load of 50N, a speed of 300 cpm, and a time of 60 minutes.

Test results (Table 1) shown for the wear length, are values relative to a value of 1 set for CrN film prepared as the comparative example. The shorter the wear length, the better the resistance to wear becomes.

The crack resistance and peeling resistance tests were performed using the VDH testing machine shown in FIG. 4. A load P was applied to a test piece 21 of about 1 to 2 centimeters cut from a piston ring and the test piece 21 was pressed against a rotor 22 rotating at a fixed speed to make scuffing occur. After operating the testing machine in this state for a fixed period of time, the sliding surface was observed for cracks or peeling. Testing was performed a number of times while varying the load. A tubing pump or an air dispenser supplied the lubricating oil. Test conditions were a speed of 1,000 rpm, a time of 1 minute, and an initial load of 40N that was gradually increased until cracks or peeling occurred.

Test results (Table 1) for the crack-peeling load, are values relative to a value of 1 set for CrN film prepared as the comparative example. The higher the crack-peeling load, the better the resistance to crack and peeling becomes.

	TABLE 1
						Amorphous		Crack-
			Si	Al	O, C	phase	Wear	Peeling
	No.	Film type	at. %	at. %	at. %	Area %	length	load
Embodiment	1	Cr—Si—N	1.0	—	—	0	0.9	1.1
	2		3.9	—	—	0	0.6	2.0
	3		9.5	—	—	2.1	0.5	1.4
	4	Cr—Si—Al—N	3.6	1.4	—	1.1	0.6	1.9
	5		9.0	6.2	—	4.5	0.6	1.2
	6	Cr—Si—O—N	4.1	—	O: 3.5	1.2	0.6	1.9
	7	Cr—Si—C—N	3.2	—	C: 8.1	1.2	0.9	1.2
Comparative	1	CrN	—	—	—	0	1.0	1.0
Example	2	Cr—Si—N	0.6	—	—	0	1.0	1.0
	3		10.0	—	—	2.3	0.7	0.9
	4		13.8	—	—	5.0	1.0	0.8
	5		15.3	—	—	6.7	1.1	0.7
	6	Cr—Si—Al—N	9.1	7.5	—	7.0	1.1	0.7
	7		8.2	23.1	—	45.0	2.5	0.5
	8	Cr—Si—C—N	3.5	—	C: 11.2	2.6	0.7	0.8

Results from Table 1 reveal the following.

(1) The wear resistance, the crack resistance and peeling resistance were both improved at a silicon content of 1 percent or more in the hard film (embodiment 1), and an especially strong effect was observed (embodiment 2).

(2) The crack resistance and peeling resistance deteriorated somewhat at a silicon content of 9.5 percent but was still superior to the CrN film (embodiment 3).

(3) Adding a small quantity of aluminum yielded the same effect as when no aluminum was added to the film (embodiment 4). However, the crack resistance and peeling resistance tend to deteriorate when the content was in the vicinity of 7 percent but was superior to the CrN film (embodiment 5).

(4) Even when a small quantity of oxygen was added to the film, the same effect was obtained as when no oxygen was added (embodiment 6).

(5) When the carbon content added to the film approached 10 percent, the crack resistance and peeling resistance tend to deteriorate compared to film where no carbon was added, however, the crack resistance and peeling resistance was superior to the CrN film (embodiment 7).

(6) When the silicon content in the hard film was lower than 1 percent at an atomic ratio, no superiority in wear resistance, crack resistance and peeling resistance was found compared to the CrN film (comparative example 2).

(7) When the silicon content was 10 percent, the resistance to wear remained at a high level but the crack resistance and peeling resistance deteriorated (comparative example 3).

(8) When the silicon content increased even further and the amorphous phase ratio reached 5 percent, the crack resistance and peeling resistance deteriorated even more (comparative example 4).

(9) When the silicon content increased even further and the amorphous phase ratio exceeded 5 percent, the wear resistance also deteriorated as well as the crack resistance and peeling resistance (comparative example 5).

(10) When the aluminum additive content exceeded 7 percent, the formation of amorphous phase was accelerated, and the wear resistance, the crack resistance and peeling resistance all deteriorated (comparative examples 6, 7).

(11) When the carbon additive content exceeded 10 percent, the resistance to wear was excellent but the crack resistance and peeling resistance deteriorated (comparative example 8).

In the examples described in the above embodiments, the hard film of the present invention covered only the outer circumferential surface of the piston ring. However, the present invention is not limited to these examples and besides the outer circumferential surface of the piston ring, the hard film of the present invention may cover the upper and lower surfaces and the inner circumferential surface.

Claims

1. A piston ring for internal combustion engines comprising a hard film formed on at least an outer circumferential sliding surface of the piston ring, wherein the hard film includes chromium, nitrogen and silicon as structural elements, and the same crystal structure as CrN, and is composed of a crystal phase containing silicon in a solid solution state in a crystal lattice at an atomic ratio between 1 and 9.5 percent.

2. A piston ring for internal combustion engines comprising a hard film formed on at least an outer circumferential sliding surface of the piston ring, wherein the hard film is composed of a mixed phase of a crystal phase and an amorphous phase,the crystal phase includes chromium, nitrogen and silicon as structural elements, and the same crystal structure as CrN, and moreover includes silicon contained in a solid solution state in a crystal lattice,the amorphous phase includes silicon, nitrogen and chromium as structural elements,the ratio of the amorphous phase in the hard film is 4.5 percent or less, andthe silicon content in the hard film is between 1 and 9.5 percent at an atomic ratio.

3. A piston ring for internal combustion engines as claimed in claim 1, wherein the hard film includes one or more types of metallic elements selected from among aluminum, vanadium, titanium and zirconium as structural elements; and the one or more metallic elements are contained in a solid solution state in a crystal lattice, and the element content is 7 percent or less at an atomic ratio.

4. A piston ring for internal combustion engines as claimed in claim 2, wherein the hard film includes one or more types of metallic elements selected from among aluminum, vanadium, titanium and zirconium as structural elements, and the one or more metallic elements are contained as the amorphous phase structural elements as well as being contained in a solid solution state in a crystal lattice of the crystal phase; and the element content is 7 percent or less at an atomic ratio.

5. A piston ring for internal combustion engines as claimed in claim 1, wherein the hard film includes one or more types of elements selected from among boron, carbon, oxygen and fluorine as structural elements; and the one or more elements are contained in a solid solution state in a crystal lattice, and the element content is 10 percent or less at an atomic ratio.

6. A piston ring for internal combustion engines as claimed in claim 3, wherein the hard film includes one or more types of elements selected from among boron, carbon, oxygen and fluorine as structural elements; and the one or more elements are contained in a solid solution state in a crystal lattice, and the element content is 10 percent or less at an atomic ratio.

7. A piston ring for internal combustion engines as claimed in claim 2, wherein the hard film includes one or more types of elements selected from among boron, carbon, oxygen and fluorine as structural elements; and the one or more elements are contained as the amorphous phase structural elements as well as being contained in a solid solution state in a crystal lattice of the crystal phase; and the element content is 10 percent or less at an atomic ratio.

8. A piston ring for internal combustion engines as claimed in claim 4, wherein the hard film includes one or more types of elements selected from among boron, carbon, oxygen and fluorine as structural elements; and the one or more elements are contained as the amorphous phase structural elements as well as being contained in a solid solution state in a crystal lattice of the crystal phase; and the element content is 10 percent or less at an atomic ratio.